Bump flat-cache to ^6.x (fix inflight vulnerability)

[zalesky]

Tested on node v22.16.0 (npm v10.9.2)
Closes #93

This PR updates the affected package to address a known security vulnerability, ensuring it is safe to use within the project.

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🔒 Security Information
• Vulnerability type: Missing Release of Resource after Effective Lifetime
• Severity:
• Snyk CVSS v3.1: 6.2 (Medium)
• NVD: No CVE assigned yet
• Source: Snyk

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🧠 Details

The vulnerability is caused by the makeres function failing to properly delete keys from the reqs object after callbacks are executed. This leads to a memory leak which can cause resource exhaustion, resulting in a Node.js process crash.
• The issue exists in a non-maintained library
• There is no official fix
• Several dependent packages have resolved the issue by removing the dependency

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✅ Resolution

This PR updates the dependency to a safe or patched version (or eliminates the vulnerable package if that’s the case), which resolves the issue and allows continued use without exposure to the vulnerability.

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🛡️ Impact
• Fixes memory leak vulnerability
• Reduces risk of application crash due to resource exhaustion
• Required for safe package usage in production

[zalesky]

@hipstersmoothie @Cloud2Software Please review and merge this PR.
This vulnerability prevents downstream projects from safely using the package.
If possible, publishing a new version after merging would be greatly appreciated. 🙌