Default enable FF bitbucket_server_parallel_importer

What does this MR do and why?

In !120931 (merged) we introduced the bitbucket_server_parallel_importer feature flag. This MR enables the bitbucket_server_parallel_importer FF by default.

Screenshots or screen recordings

N/A

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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