jwt: replace deprecated `CI_JOB_JWT` variable with an ID token

See https://docs.gitlab.com/ee/update/deprecations.html#old-versions-of-json-web-tokens-are-deprecated

Second attemp for a241d213. I had to revert the original commit to prevent pipelines of other projects failing. The JWT authorizer on the docker registry hosts hadn't been restarted and was still running the old code

Bug: T337474