Backport legacy Group inner wrapper fix

[tellthemachines]

Trac ticket: https://core.trac.wordpress.org/ticket/56467

This PR backports changes from WordPress/gutenberg#44660 to fix legacy Group block inner containers.

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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.

[noisysocks]

Looks good 👍

Let's make sure this is committed for RC 2 cc. @ockham @hellofromtonya.

[ockham]

Testing as follows:

• On trunk, switch to a Classic Theme (e.g. TT1).
• Create a new post, and insert the Group block.
• Inside the Group block, insert the Columns block. Choose e.g. a 33/66 two-column layout.
• In each column, insert a paragraph block, and enter some text.
• Publish the post, and view on the frontend.
• Check the page source for the Group block: There's no div with class wp-block-group__inner-container.

Then, switch to this branch, re-install npms and rebuild.

• Reload the post. While there is no visual difference compared to trunk, the page source now has a div with class wp-block-group__inner-container.

trunk	This branch

A quick search of wp-block-group__inner-container in this repo reveals that it is indeed used for styling in Classic Themes, e.g. in Twenty Nineteen, Twenty Twenty, or Twenty Twenty One ✅

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@tellthemachines Please don't forget to include testing instructions in PRs like this. I'm not very familiar with the Group block myself, and testing instructions in the originating Gutenberg PR and issue were rather sparse, and they didn't make it abundantly clear why this is an issue that warrants a fix during the RC phase.

It'd be great to have step-by-step instructions and before/after visuals that illustrate how badly this would break existing themes -- ideally in a more compelling way than my little frontend page source screenshots.

[dream-encode]

I am happy to merge this since it's a very tightly scoped fix and appears to remedy a bug introduced during the 6.1 cycle. I agree with @ockham that I'd like to see some more testing to ensure this resolves the known issue, but, more importantly, doesn't have any unintended side effects.

[tellthemachines]

@tellthemachines Please don't forget to include testing instructions in https://github.com/WordPress/gutenberg/pull/44660s like this. I'm not very familiar with the Group block myself, and testing instructions in the originating Gutenberg PR and WordPress/gutenberg#44636 were rather sparse, and they didn't make it abundantly clear why this is an issue that warrants a fix during the RC phase.

Ok, but this is not usual procedure for backports, and please remember this is only being committed during RC because it was left out of the pre-RC backports even though it was linked to the correct Trac ticket.

[ockham]

[Committer double signoff]

Approving. I've tried to cover both block themes and legacy themes, post editor and site editor, and frontend, and nothing stood out to me as broken.

[dream-encode]

Merged into core in https://core.trac.wordpress.org/changeset/54633.