Full Site Editing: Replace temporary template block injection from core/template to core/group

[Copons]

Changes proposed in this Pull Request

The core/template block will be removed in Gutenberg 6.5. (Props to @glendaviesnz for finding out about this!)
Full Site Editing uses it to group the content (blocks) of FSE templates before feeding them to their BlockEdit components inside the page editor.

This PR simply uses core/group as grouping block instead of core/template.

After some tests, it doesn't seem to me there are any differences with the previous approach, but I wasn't around when we decided to use core/template, and there might be edge cases (e.g. with the template locking, which is not clear to me how it works) that I haven't considered.

Testing instructions

• On an FSE test site, update Gutenberg to 6.5 (or build from master).
• Try opening an FSE page, and observe that the template blocks are broken because the core/template block doesn't exist anymore.
• Apply this PR.
• Reload the FSE page (make sure it didn't autosave without the template blocks, in which case restore a previous revision).
• Make sure the template blocks look as expected.
• Save and view the page in the front end.
• Make sure the page looks as expected.

Fixes #36217

[matticbot]

Test live: https://calypso.live/?branch=fix/fse-core-template-block

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[gwwar]

Tests well for me @Copons, sandboxed and testing on horizon!

Before:

After:

[noahtallen]

Looking good to me! I tested on a wpcom simple site. Editor and front end all work as expected.

[noahtallen]

I just noticed this when I was testing a sandboxed site on Horizon (i.e. has the new Gutenberg version, and I'm syncing this branch):

Unsure if related. Investigating. 👀

Edit: nevermind. This was an issue with my sync. Everything looking good :)