Enable third-party UCM flags by default

What does this MR do and why?

This MR sets bitbucket_server_user_mapping and gitea_user_mapping to be enabled by default for self-managed instances.

The paired flag, importer_user_mapping was enabled by default in !175371 (merged)

The flag for GitHub was enabled by default in !176633 (merged)

Changelog: changed

References

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• MR to enable UCM by default: !175371 (merged)
• Rollout issue for bitbucket_server_user_mapping FF: #509897 (closed)
• Rollout issue for this gitea_user_mapping FF: #498390 (closed)

MR acceptance checklist

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