Enable virtual_registry_maven_cleanup_new_worker_class by default

Context

In !177996 (merged), we finalized a pretty deep refactoring for Maven virtual registries. We introduced a new (partitioned) table for the caching aspect and in that MR, we updated all models to reference the new table.
Among the code changes, there was this worker that needed to be renamed. Worker class renames should follow https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#renaming-worker-classes.
This MR is step two of that process: enable the feature flag by default. So the overall progress on that process in #515213 (closed).
Note that the maven virtual registry is gated behind a wip
feature flag. Thus, the feature flag changed here (the one for the worker class rename) is a feature flag within a feature flag.

What does this MR do and why?

- Enable virtual_registry_maven_cleanup_new_worker_class by default

References

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MR acceptance checklist

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Screenshots or screen recordings

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How to set up and validate locally

Follow !165195 (merged).