Clear CI caching entries from closed PRs

Cache entries from closed PRs are only removed automatically after 7 days (https://docs.github.com/en/actions/reference/workflows-and-actions/dependency-caching#default-limits). The cache limit has been increased from the default 10 GB to 45 GB (#5318) but it might be still worth clearing old entries explicitly as soon as possible. Refer to implementation in #5319 and split it into its own PR.

Read the implementation referenced in issue #5319 and the GitHub Actions dependency-caching documentation linked in the issue. Separate that implementation into its own change, then verify that cache entries from closed pull requests are cleared as soon as possible rather than after the seven-day automatic period.

Written by the indexing model from the issue text.