run CI on all PR branches (#282)

This is a follow-up to #280 (comment). In summary, we only run CI checks on PRs against the main branch. However, in cases where an older major version of a package needs to be patched, it doesn't make sense to PR against main. CI won't run for these PRs, but it probably should.

This updates our GitHub CI configs to run checks against PRs against any branch.

---

<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor guide](https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md) for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before creating a PR.
- Contributions to our repos should follow the [Dart style guide](https://dart.dev/guides/language/effective-dart) and use `dart format`.
- Most changes should add an entry to the changelog and may need to [rev the pubspec package version](https://github.com/dart-lang/sdk/blob/main/docs/External-Package-Maintenance.md#making-a-change).
- Changes to packages require [corresponding tests](https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md#Testing).

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.
</details>

Author: andrewkolos

.github/workflows/cli_config.yml

@@ -3,7 +3,6 @@ permissions: read-all
 
 on:
   pull_request:
-    branches: [main]
     paths:
       - ".github/workflows/cli_config.yml"
       - "pkgs/cli_config/**"

.github/workflows/extension_discovery.yml

@@ -3,7 +3,6 @@ permissions: read-all
 
 on:
   pull_request:
-    branches: [ main ]
     paths:
       - '.github/workflows/unified_analytics.yml'
       - 'pkgs/extension_discovery/**'

.github/workflows/graphs.yml

@@ -2,13 +2,12 @@ name: package:graphs
 permissions: read-all
 
 on:
-  # Run on PRs and pushes to the default branch.
-  push:
-    branches: [ main ]
+  # Run CI on all PRs (against any branch) and on pushes to the main branch.
+  pull_request:
     paths:
       - '.github/workflows/graphs.yml'
       - 'pkgs/graphs/**'
-  pull_request:
+  push:
     branches: [ main ]
     paths:
       - '.github/workflows/graphs.yml'

.github/workflows/unified_analytics.yml

@@ -3,7 +3,6 @@ permissions: read-all
 
 on:
   pull_request:
-    branches: [ main ]
     paths:
       - '.github/workflows/unified_analytics.yml'
       - 'pkgs/unified_analytics/**'