Increase the CI cache key file limit to 10

What does this MR do and why?

Raises the cache:key:files limit from 2 to 10, behind the increase_ci_cache_key_files_limit feature flag (beta, default off).

Performance note: literal paths are resolved in one batched Gitaly call. Each wildcard pattern costs its own SearchFilesByName Gitaly call (a full tree listing), so the worst case goes from 2 such calls per job cache entry to up to 10. The flag allows a gradual rollout while watching Gitaly load.

References

Relates to #301161

Rollout issue: #631277

Screenshots or screen recordings

How to set up and validate locally

1. With feature branch checked out, enable feature flag increase_ci_cache_key_files_limit

2. In any project, go to Build > Pipeline editor and paste this config:

   cache-job:
     cache:
       key:
         files:
           - file1
           - file2
           - file3
           - file4
           - file5
           - file6
           - file7
           - file8
           - file9
           - file10
       paths:
         - vendor/ruby
     script: echo "test"

3. Expected: Validate reports the config as valid, and the editor shows no schema warning on the files list.

4. Add an 11th file (file11). Expected: validation fails with jobs:cache-job:cache:key:files config has too many items (maximum is 10), and the editor underlines the list.

5. Optional: run the same checks in CI Lint at <project>/-/ci/lint.

6. The 10-file config fails when feature flag is off (maximum is 2).

MR acceptance checklist

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Related to #301161