Default enable webauthn feature flag

What does this MR do?

This MR default enables the webauthn feature flag, following the iteration plan for introducing WebAuthn support as outlined in #22506 (comment 298336500) and &4061

Note: There is still a (more-or-less) ongoing discussion in the previous MR (#46179) as some old U2F registrations have not been successfully migrated to WebAuthn on GitLab.com. If there is something we can do to fix this, is might be necessary to do that first before merging this MR. However, at the moment, I don't see any way to further analyze / fix this issue.

Screenshots (strongly suggested)

(See previous MRs, as #26692)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #232671 (closed)