[Gutenberg] - Remove Gallery V1 and experimental flags

[geriux]

Related PRs:

• Remove Gallery Block V1 and other experimental flags gutenberg-mobile#6973
• Gallery Block: Cleanup v1 code WordPress/gutenberg#63285

To test:

• Test adding a Gallery block to a post

Regression Notes

1. Potential unintended areas of impact
   It affects only the editor.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manual testing and existing automated tests for the Gallery block.

3. What automated tests I added (or what prevented me from doing so)
   No tests were added since this is only removing the old Gallery block format and a few experimental flags.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 25.2. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

Generated by 🚫 Danger

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr23411-cc36e3e
	Version	25.1
	Bundle ID	com.jetpack.alpha
	Commit	cc36e3e
	App Center Build	jetpack-installable-builds #9335

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23411-cc36e3e
	Version	25.1
	Bundle ID	org.wordpress.alpha
	Commit	cc36e3e
	App Center Build	WPiOS - One-Offs #10288

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[dcalhoun]

Test plan succeeded for me. I noted oddities I observed in wordpress-mobile/gutenberg-mobile#6973 (review).