Add deprecation notes for JWT CI/CD variable

What does this MR do?

The CI_JOB_JWT, CI_JOB_JWT_V1 and CI_JOB_JWT_V2 were all deprecated and scheduled to be removed in GitLab 16.0. We have a deprecation notice, but there are still some docs referencing this. It's going to take time to update those processes to use ID tokens instead, but for now we should add deprecation notes to those docs and point people to the ID tokens docs.

Related issues

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