Make CI cache limit per job configurable by admins

What does this MR do and why?

Replaces the hardcoded MULTIPLE_CACHE_LIMIT = 4 in Gitlab::Ci::Config::Entry::Caches with a new ci_max_caches_per_job application setting, so instance administrators can raise the limit when their use case requires more than four cache: entries in a single CI/CD job.

• Default: 4 — preserves existing behavior on every instance.
• Minimum: 1 — enforced by model validation (numericality: { only_integer: true, greater_than: 0 }) and the HTML min attribute on the input.
• Where: Admin Area → Settings → CI/CD → Continuous Integration and Deployment → Maximum caches per job.

The setting reuses the existing ci_cd_settings_definition JSONB column, so no database migration is required. GitLab.com is unaffected unless an admin explicitly changes the value.

References

• Original issue: #421962

How to set up and validate locally

1. As an admin, visit Admin Area → Settings → CI/CD and expand Continuous Integration and Deployment. Confirm a new Maximum caches per job field appears below Maximum includes.

2. With the setting left at the default 4, create a project with a .gitlab-ci.yml that declares 5 caches in a single job and confirm validation rejects it with no more than 4 caches can be created:

   test:
     cache:
       - { key: a, paths: [a/] }
       - { key: b, paths: [b/] }
       - { key: c, paths: [c/] }
       - { key: d, paths: [d/] }
       - { key: e, paths: [e/] }
     script: echo "hello"

3. Raise Maximum caches per job to 5 and save. Re-validate the same config (CI Lint or pipeline editor) and confirm it now passes.

4. Try entering 0 in the field and confirm both browser and server-side validation reject the value.

Related to #421962