Editor: Fix post type check in isEditingTemplate selector

[mattgrshaw]

What?

This PR fixes what seems to be an invalid check in isEditingTemplate that was causing it to return true, even when not editing a template.

Why?

ACF was using this selector to check if a template was currently being edited instead of a standard post. It looks like this has been deprecated, but in WP 6.5-beta1 it seems to always return true, which could cause issues for plugins that haven't updated their code.

How?

isEditingTemplate now checks for the wp_template post type instead of post-only.

Testing Instructions

1. In the current 6.5 beta, check the output of the isEditorTemplate selector in a standard post or page. It seems to always return true.
2. Repeat the same on this branch, noting that it now returns false, but still returns true when editing a template (via the sidebar -> Post -> Template dropdown -> Create new template)

Screenshots or screencast

[github-actions]

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[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @mattgrshaw! In case you missed it, we'd love to have you join us in our Slack community.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.

[youknowriad]

Yes, sorry, my mistake here.

[annezazu]

Seeing approvals for this PR and that it's ready to squash and merge. Proceeded with doing so in order to get into 6.5!

[getdave]

I just cherry-picked this PR to the cherry-pick-wp-6-5-beta-3 branch to get it included in the next release: 8646cd1