CustomSelectControlV2: do not flip popover if legacy adapter

[ciampo]

What?

Part of #55023

Do not allow the select popover in the CustomSelectControl V2 legacy adapter to flip when there isn't much space to render below the trigger button.

Why?

This matches more closely the behavior of the legacy, downshift-based component and prevents (avoids it without solving) the issue reported in #63180

How?

By setting the flip property to false on the ariakit select popover

Testing Instructions

• Render CustomSelectControlV2 legacy adapter and open the select popover
• Resize its window until there is not enough space for the popover
• Notice how the popover doesn't flip, but continues to render below the trigger button

Screenshots or screencast

Before (trunk)	After (this PR)
Kapture.2024-07-10.at.16.32.46.mp4	Kapture.2024-07-10.at.16.36.45.mp4

[github-actions]

Flaky tests detected in 061b9e2.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/9887447341
📝 Reported issues:

• [Flaky Test] Pagination links are working as expected #39755 in /test/e2e/specs/editor/blocks/comments.spec.js

[github-actions]

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Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: DaniGuardiola <daniguardiola@git.wordpress.org>
Co-authored-by: tyxla <tyxla@git.wordpress.org>

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[DaniGuardiola]

Tests well for me!

[tyxla]

I like this 👍