Real-time collaboration: change to opt-in (off-by-default).

See: https://wordpress.slack.com/archives/C07NVJ51X6K/p1773850504196589.

We are intentionally changing the option name so that it will be off-by-default for everyone, including those that installed a beta release where the feature was on-by-default.

Developed in: https://github.com/WordPress/wordpress-develop/pull/11289.

Fixes #64845.
Props czarate, peterwilsoncc, jorbin.

git-svn-id: https://develop.svn.wordpress.org/trunk@62058 602fd350-edb4-49c9-b593-d223f7449a82