Template activation: initialise old autosave and revisions endpoints

[ellatrix]

What?

Syncs back PHP changes in core from WordPress/wordpress-develop#8063. See #71735.

Why?

The old autosave and revisions endpoints must be initialized for backwards compatibility.

How?

Testing Instructions

Use the endpoints. There are unit tests for this in core.

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: ellatrix <[email protected]>

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[ellatrix]

I'm going to backport this just to make cherry picking easier. It has no effect on the packages.

[github-actions]

I just cherry-picked this PR to the wp/6.9 branch to get it included in the next release: ddc8431

[github-actions]

Warning: Type of PR label mismatch

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• Labels found: [Feature] Templates API, [Type] Code Quality, Backport from WordPress Core, Backported to WP Core, No Core Sync Required.

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