Log a warning when JWT cannot be decoded in dependency proxy auth

What does this MR do and why?

Some users have experienced intermittent authentication failures when using the dependency proxy. A 401 might be returned if the JWT token can't be decoded for some reason. Previously we quietly discarded the token, but this commit adds a log message to track the frequency of this.

Relates to #332827

How to set up and validate locally

It's a bit surprising to me the auth endpoint uses /v2: https://gitlab.com/gitlab-org/gitlab/-/blob/ee513ee6b441894c0d68bc96d8891f547a45e7a2/config/routes/group.rb#L163

You can test this by injecting some arbitrary JWT:

curl -v -X GET -H "Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c" https://host.docker.internal:3443/v2 

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