What?

Ensure that fallback gap styles are only output for the flex layout type, so that in Classic themes (or themes that do not opt-in to block gap) we do not output the gap styles for the constrained layout type.

Why?

In #42763 we didn't factor in that the existing logic for fallback gap was outputting fallback gap styles for layout types that are not default. Because the constrained layout type behaves in much the same way as the default layout type, we need to update the logic for the fallback gap so that it only applies to the flex layout type. The reason the fallback exists is primarily so that blocks like Buttons, Social Icons, and Columns render with a pleasing default gap. Vertically oriented blocks using the default and constrained layout types do not need the same kind of fallback gap to be output.

How?

Update the checks for default layout type that skip block gap output to instead be "not flex" instead.

Testing Instructions

Here is some test markup for looking at blocks within a constrained layout type:

<!-- wp:group {"layout":{"type":"constrained"}} -->
<div class="wp-block-group"><!-- wp:paragraph -->
<p>A paragraph in a group</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Another paragraph in a group</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>And another one</p>
<!-- /wp:paragraph --></div>
<!-- /wp:group -->

In a Classic theme, prior to this PR, note that low specificity vertical gap (top margin) styles are being output in the editor (and also on the front end, but usually they're overridden by something with higher specificity).

With this PR applied, the constrained layout gap rules should no longer be output.

With blocks-based themes that use block gap (e.g. TwentyTwentyTwo), the gap should be the same as prior to this PR.

Screenshots or screencast

Before (note that constrained layout gap rules are applied)	After (note that the constrained layout gap rules no longer exist, the classic.css rules are applied for margins)