MU WPCOM: Enable Gutenberg RTC for sites with the REAL_TIME_COLLABORATION feature

[arthur791004]

Related to DOTCOM-16207

Proposed changes

Enable the Gutenberg RTC (Real-Time Collaboration) feature for sites with the REAL_TIME_COLLABORATION feature flag, instead of being disabled by default for all sites.

• Update wpcom_is_gutenberg_rtc_enabled() to check for the REAL_TIME_COLLABORATION feature using wpcom_site_has_feature() and get_wpcom_blog_id().
• The result is still filterable via the wpcom_is_gutenberg_rtc_enabled filter, so it can be overridden per-site if needed.

Other information

• Generate changelog entries for this PR (using AI).

Related product discussion/links

Does this pull request change what data or activity we track or use?

No.

Testing instructions

0. Apply changes to your WoA dev site by Jetpack Beta Tester
1. Ensure the test site has the REAL_TIME_COLLABORATION feature enabled. See Gutenberg RTC: Add real_time_collaboration feature #47483
2. Edit a page/post
3. Ensure the value of window._wpCollaborationEnabled is true
4. On a site without the feature, verify that Gutenberg RTC remains disabled.
5. Ensure the value of window._wpCollaborationEnabled is undefined