RTC: Remove post list lock icon and replace user-specific lock text

[pkevan]

Backport of Gutenberg PR #76322. When real-time collaboration is enabled, the post list should not show an exclusive lock icon or user-specific lock text since multiple users can collaboratively edit the same post. Replaces user-specific data with a generic "Currently being edited" message, hides lock UI elements, changes "Edit" to "Join" for locked posts, and re-enables controls that core normally hides.

Trac ticket: https://core.trac.wordpress.org/ticket/11234

Use of AI Tools: This PR was created with assistance from Claude (Anthropic).

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Props paulkevan, maxschmeling.

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[maxschmeling]

I'm not an expert on the approach for these types of things in core, but isn't it more appropriate in core to go and change the place that renders the lock and the text rather than filter the values and add override styles?