Enable show_container_registry_tag_signatures feature flag by default

Daniel Tianrequested to merge
460226-enable-show-container-registry-tag-signatures-feature-flag-by-default into master

What does this MR do and why?

This MR enables the show_container_registry_tag_signatures feature flag by default. It has been enabled globally on production for over a week now with no issues, and we feel confident enough to enable it by default for self-managed.

Related to #460226 (closed)

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