WebServer - change occurrence of client.flush() to clear()

[JAndrassy]

as follow up of changing flush() to clear() in networking with PR #9453, there is an unclear use of flush() in WebServer.

in Parsing.cpp the client.flush() is at the end of the processing of the HTTP request. the response is already sent. If the connection is not kept alive for the next request, then it is closed so why clearing it. If the connection is kept alive for the following HTTP request, then clear() could delete beginning of the following request. But characters remaining in the buffer would make the following request invalid too. So for a reused connection the processing of the request should ensure to read until the exact end of the request. I don't know if that is fulfilled.

Anyway client.flush() did clear the receive buffer in previous versions so this would return it to that.

[github-actions]

	Messages
📖	🎉 Good Job! All checks are passing!

👋 Hello JAndrassy, we appreciate your contribution to this project!

---

Click to see more instructions ...

This automated output is generated by the PR linter DangerJS, which checks if your Pull Request meets the project's requirements and helps you fix potential issues.

DangerJS is triggered with each push event to a Pull Request and modify the contents of this comment.

Please consider the following:
- Danger mainly focuses on the PR structure and formatting and can't understand the meaning behind your code or changes.
- Danger is not a substitute for human code reviews; it's still important to request a code review from your colleagues.
- To manually retry these Danger checks, please navigate to the Actions tab and re-run last Danger workflow.

Review and merge process you can expect ...

We do welcome contributions in the form of bug reports, feature requests and pull requests.

1. An internal issue has been created for the PR, we assign it to the relevant engineer.
2. They review the PR and either approve it or ask you for changes or clarifications.
3. Once the GitHub PR is approved we do the final review, collect approvals from core owners and make sure all the automated tests are passing.
- At this point we may do some adjustments to the proposed change, or extend it by adding tests or documentation.
4. If the change is approved and passes the tests it is merged into the default branch.

Generated by 🚫 dangerJS against e21f973

[github-actions]

Test Results

 56 files   -  83   56 suites   - 83   5m 30s ⏱️ - 1h 37m 39s
 21 tests  -   9   21 ✅  -   9  0 💤 ±0  0 ❌ ±0 
135 runs   - 168  135 ✅  - 168  0 💤 ±0  0 ❌ ±0 

Results for commit e21f973. ± Comparison against base commit cd3d0bf.

This pull request removes 9 tests.

performance.coremark.test_coremark ‑ test_coremark
performance.fibonacci.test_fibonacci ‑ test_fibonacci
performance.psramspeed.test_psramspeed ‑ test_psramspeed
performance.ramspeed.test_ramspeed ‑ test_ramspeed
performance.superpi.test_superpi ‑ test_superpi
test_touch_errors
test_touch_interrtupt
test_touch_read
validation.periman.test_periman ‑ test_periman

♻️ This comment has been updated with latest results.

[github-actions]

Memory usage test (comparing PR against master branch)

The table below shows the summary of memory usage change (decrease - increase) in bytes and percentage for each target.

Memory	FLASH [bytes]		FLASH [%]		RAM [bytes]		RAM [%]
Target	DEC	INC	DEC	INC	DEC	INC	DEC	INC
ESP32S3	0	⚠️ +24	0.00	0.00	0	0	0.00	0.00
ESP32S2	0	⚠️ +32	0.00	0.00	0	0	0.00	0.00
ESP32C3	0	⚠️ +36	0.00	0.00	0	0	0.00	0.00
ESP32C6	0	⚠️ +36	0.00	0.00	0	0	0.00	0.00
ESP32	0	⚠️ +36	0.00	0.00	0	0	0.00	0.00

Click to expand the detailed deltas report [usage change in BYTES]

Target	ESP32S3		ESP32S2		ESP32C3		ESP32C6		ESP32
Example	FLASH	RAM	FLASH	RAM	FLASH	RAM	FLASH	RAM	FLASH	RAM
WebServer/examples/AdvancedWebServer	⚠️ +24	0	⚠️ +24	0	⚠️ +34	0	⚠️ +34	0	⚠️ +32	0
WebServer/examples/FSBrowser	⚠️ +24	0	⚠️ +24	0	⚠️ +34	0	⚠️ +34	0	⚠️ +28	0
WebServer/examples/Filters	⚠️ +24	0	⚠️ +28	0	⚠️ +34	0	⚠️ +34	0	⚠️ +28	0
WebServer/examples/HelloServer	⚠️ +24	0	⚠️ +28	0	⚠️ +36	0	⚠️ +36	0	⚠️ +32	0
WebServer/examples/HttpAdvancedAuth	⚠️ +24	0	⚠️ +32	0	⚠️ +36	0	⚠️ +34	0	⚠️ +28	0
WebServer/examples/HttpAuthCallback	⚠️ +24	0	⚠️ +24	0	⚠️ +34	0	⚠️ +34	0	⚠️ +28	0
WebServer/examples/HttpAuthCallbackInline	⚠️ +24	0	⚠️ +32	0	⚠️ +34	0	⚠️ +34	0	⚠️ +28	0
WebServer/examples/HttpBasicAuth	⚠️ +24	0	⚠️ +28	0	⚠️ +34	0	⚠️ +36	0	⚠️ +36	0
WebServer/examples/HttpBasicAuthSHA1	⚠️ +24	0	⚠️ +32	0	⚠️ +34	0	⚠️ +34	0	⚠️ +28	0
WebServer/examples/HttpBasicAuthSHA1orBearerToken	⚠️ +12	0	⚠️ +24	0	⚠️ +34	0	⚠️ +34	0	⚠️ +32	0
WebServer/examples/MultiHomedServers	⚠️ +24	0	⚠️ +24	0	⚠️ +34	0	⚠️ +34	0	⚠️ +28	0
WebServer/examples/PathArgServer	⚠️ +24	0	⚠️ +24	0	⚠️ +34	0	⚠️ +34	0	⚠️ +20	0
WebServer/examples/SDWebServer	⚠️ +24	0	⚠️ +24	0	⚠️ +34	0	⚠️ +34	0	⚠️ +28	0
WebServer/examples/SimpleAuthentification	⚠️ +24	0	⚠️ +24	0	⚠️ +34	0	⚠️ +34	0	⚠️ +32	0
WebServer/examples/UploadHugeFile	⚠️ +24	0	⚠️ +28	0	⚠️ +34	0	⚠️ +34	0	⚠️ +24	0
WebServer/examples/WebServer	⚠️ +24	0	⚠️ +24	0	⚠️ +34	0	⚠️ +34	0	⚠️ +28	0
WebServer/examples/WebUpdate	⚠️ +24	0	⚠️ +28	0	⚠️ +34	0	⚠️ +34	0	⚠️ +28	0