Disable IAT verification by default

What does this MR do and why?

!117468 (merged) in GitLab 15.11 updated the ruby-jwt gem to v2.5.0. In v2.2.0, ruby-jwt removed the iat_leeway parameter (https://github.com/jwt/ruby-jwt/pull/274).

As a result, if a gitlab-shell host creates a JWT token with an issued-at (IAT) claim that is slightly behind the host handling API the request, users will receive a 401 error.

Disable this IAT verification by default since it's not serving a useful purpose, since expiration times are already validated. We already made a similar change in Geo.

Relates to #417543 (closed)

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