Fix alignment of notice lists in block placeholders

[p-jackson]

Description

This change updates the margins of the notice UI added to block placeholders using the withNotices mixin. The update was necessary after #18745 which made the content of block placeholders left-aligned.

Fixes #19671

How has this been tested?

• Get the File block into error state by uploading an invalid file and then look at the rendered output to observe any layout issues.
• Tested with File block in column layout
• Tested mobile width
• Safari, FF and Chrome on Mac
• Viewed the fix in a testing block with no other content but the placeholder. This was to check that the fix worked in general, not just for the File block.

Screenshots

Before #18745
Block placeholder content is centred.

After #18745
With alignment bug as reported by #19671

After this fix
Vertical margins the same as before #18745, but notice is now full width

Types of changes

Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[epiqueras]

I don't think we should remove the set width.

[p-jackson]

Thanks for taking a look at this @epiqueras :)

I don't think we should remove the set width.

To my eye at least the full width notice looks a little nicer than having the extra 40px on the right:

It's so close to full width that IMO it really feels like it should be anchored to the right-hand side.

I'm happy to add the width rule back if that's the layout we want though. I may have missed something that makes the extra space necessary.

[epiqueras]

@jasmussen Did you add the width there for a reason?

[jasmussen]

I'm sure there was a reason, but that reason isn't there anymore. It seems like both of the margin and width rules can be removed: