Refactor update_ignored_hooked_blocks_postmeta to use meta_input

[tjcafferkey]

This is a prerequisite to adding hooked blocks to template parts at positions first_child and last_child #6867

It's necessary because unlike wp_navigation post, the wp_template_part posts won't have a database entry upon first save meaning that update_post_meta won't work. To circumvent this we are now setting meta information on the $post stdClass object via the meta_input field which will get saved as post meta when its created/updated in the database.

Trac ticket: https://core.trac.wordpress.org/ticket/61495

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[ockham]

Some additional background here.

[ockham]

Thank you!

[ockham]

Committed to Core trunk in https://core.trac.wordpress.org/changeset/58578.