Improve consistency and accessibility of the Block placeholder variations picker

[afercia]

Description

See conversation on #60206 and #60217
Re: the importance of having consistent block placeholders, see also #59275

Blocks that require some initial input / user choice for their initial configuration use a 'placeholder' UI. The design pattern for these placeholders is made of:

• Icon + title.
• Description.
• Interactive controls for the initial configuration / choice.

Also blocks that present a 'variations picker' in the placeholder (e.g. Columns and Query Loop) use this design pattern, except the Group block.
Why?
I'm not sure this inconsistency helps users in any way and I don't see any reason why the Group placeholder should not use the same design pattern of other blocks. Screenshot:

Additionally:
The variations picker for Columns, Query Loop, and Group have some long standing accessibility problems that need to be addressed. Most notably: the visible text mismatches the actual accessible name of the controls.
Labels and descriptions are used incorrectly on these variations pickers, mostly vor visual purposes. For semantics and accessibility, all interactive controls should:

• Be labeled based on what they do, to communicat ethe control action.
• Have an optional description to provide additional information, that isn't part of the label but still correctly associated with the control (likely via aria-describedby).

Step-by-step reproduction instructions

• Add an Image block and observe its initial placeholder.
• Add a Columns block and observe its initial placeholder.
• Add a Query Loop block. Click 'Start blank' and observe the blockinitial placeholder.
• Observe these three block placeholders use a consistent design pattern.
• Add a Group block.
• Observe the placeholder with the variations picker uses an inconsistent design pattern.
• Observe the visible text below the variations isn't part of the variation button.
• Columns: observe the visible text below each variation mismatches the actual accessible name of the variation button.
• Group: observe the accessible name of the variation buttons is only exposed via a tooltip.

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

[jasmussen]

I left a comment that's relevant to this PR here: #67083 (comment)