Use JsonLogger from gitlab-labkit

What does this MR do and why?

The recent update to gitlab-labkit 0.26 includes a JsonLogger extraction from this repository. This was done in gitlab-org/ruby/gems/labkit-ruby!96 (merged). This MR updates gitlab-rails to utilize this extracted logger.

The Gitlab::JsonLogger still exists and inherits from Labkit::Logging::JsonLogger since it retains some functionality specific to gitlab-rails.

Semantically, there should be no change.

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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