Rename wp_register_block_template() to register_block_template()

[Aljullu]

This PR includes the changes from WordPress/gutenberg#65958 that need to be backported into WP core.

Trac ticket: https://core.trac.wordpress.org/ticket/62193

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Props aljullu, peterwilsoncc, aristath, youknowriad.

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[peterwilsoncc]

This looks good to me based on a read through of the code.

Should we wait for the Gutenberg PR to be merged prior to merging this or can they happening independently?

[aristath]

LGTM 👍

[youknowriad]

The Gutenberg PR is merged. Thanks for the follow-up here.

[peterwilsoncc]

Merged r59201 / 4ccb5b4

[getdave]

Thank you for the followup and for getting this committed everyone 👍