JWT issuer: allow for multiple values

Merged Jaime Nuche requested to merge mutliple-issuers into main

Recently, tokens used by GitLab changed the value in their issuer field. Supporting multiple issuers will help us deal with this scenario in the future in case it happens again.

Also changed default issuer to the one used by the new tokens: https://gitlab.wikimedia.org

Bug: T337474

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