ci: add workflow to cleanup per-PR Actions caches on close

[mr-raj12]

Adds .github/workflows/cleanup-pr-caches.yml. The workflow runs on pull_request.closed and deletes every GitHub Actions cache scoped to the closing PR's ref (refs/pull/<n>/merge) via actions/github-script.

Without this, per-PR caches linger under the 10 GB repository cache quota until GitHub's 7-day LRU eviction kicks in, and caches for master and open PRs can be evicted ahead of them. This workflow keeps the quota available for the branches that matter.

• New file, so it is outside the scope of the central asyncapi/.github workflow sync.
• Runs only on PR close — no impact on existing CI jobs.
• Scoped permissions: actions: write, contents: read.

Resolves #1155

[changeset-bot]

⚠️ No Changeset found

Latest commit: c6ab8c5

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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