Templates: Add date field

[ntsekouras]

Backports: WordPress/gutenberg#77134

Trac ticket: https://core.trac.wordpress.org/ticket/65049

This PR adds the date field in templates and template parts. This will help with the simplification of revisions handling for these post types, as there is a lot of scattered code around that working around the fact that they expose only the modified date.

Testing Instructions

1. date field should be returned through REST API
2. everything should work exactly as before - especially the templates/template parts flows since the change affects only them.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[t-hamano]

@ntsekouras, Can you submit a core ticket so we can track this feature enhancement as part of the 7.1 release?

[github-actions]

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Props ntsekouras, audrasjb, wildworks.

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[ntsekouras]

@ntsekouras, Can you submit a core ticket so we can track this feature enhancement as part of the 7.1 release?

Of course. I opened it hastily to get the changelog entry for GB and will handle this now.

--edit

Created the ticket and updated the PR's description with it.

[audrasjb]

The PR looks good to me for a Gutenberg merge when trunk is reopened for commit.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 62571
GitHub commit: 0853070

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.