Allow removing bottom margin from SelectControl

[noisysocks]

What?

Allows removing the bottom margin from SelectControl by specifying a __nextHasNoMarginBottom prop.

Why?

• Components shouldn't specify margins, @wordpress/components is slowly transitioning away from this.
• A opt-in prop means that third parties have time to adjust before we remove the margin permanently. See https://github.com/WordPress/gutenberg/blob/trunk/packages/components/CONTRIBUTING.md#deprecating-styles.
• Needed so that we can use SelectControl in Design updates to the Publish popover (DateTimePicker) #41097.

How?

BaseControl already has a __nextHasNoMarginBottom prop so the approach here is to just use that from SelectControl.

I am not adding a call to deprecate() as there are dozens of uses of SelectControl in Gutenberg that need to be adjusted first. This is also the approach that was taken with BaseControl.

Testing Instructions

1. npm run storybook:dev
2. Got to the SelectControl component.
3. Check that toggling the __nextHasNoMarginBottom removes the bottom margin from the control's wrapper.

[ciampo]

LGTM 🚀

__nextHasNoMarginBottom={ false }

__nextHasNoMarginBottom={ true }

E2E failures don't seem related.

[mburridge]

Added the Needs Dev Note label in case this needs a dev note for the 6.1 release.

[mirka]

We'll be officially deprecating the margins as a separate PR (centrally managed in #38730), at which time we'll post the Dev Note. Thanks!