Add KUBE_CA_PEM_FILE, deprecate KUBE_CA_PEM

What does this MR do?

Adds a new kubernetes deployment service variable: KUBE_CA_PEM_FILE, deprecating KUBE_CA_PEM.

The former will be written to a file by gitlab-ci-multi-runner, and the file path will be placed in the variable. The latter just puts the raw PEM data (including newlines) into an environment variable.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Currently, we're putting a multi-line string that's only ever useful as a file into an environment variable. Runner got support for putting it straight into a file and putting the filename into the variable here: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/413

Hopefully we can remove KUBE_CA_PEM at some point.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25753