Notifications: make the v3 panel the default

[arthur791004]

Proposed changes

Graduates the v3 notifications panel to the default. Previously v3 was gated behind a notifications=v3 query parameter; this drops the opt-in and always renders the v3 panel (wpnt-notes-panel3).

The query-param opt-in never shipped, so its unreleased changelog entry is updated in place rather than stacking an add + remove pair.

A small follow-up will remove the now-dead v2-panel code (e.g. inlining the constant $panel_id and dropping its esc_attr wrapper).

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• Log in to a site with the Jetpack notifications (masterbar) module active.
• Click the notifications bell in the admin bar.
• Confirm the panel container is wpnt-notes-panel3 (the v3 panel) without needing ?notifications=v3 in the URL.

🤖 Generated with Claude Code

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack), and enable the cleanup/notifications-v3-flag branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack cleanup/notifications-v3-flag

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/jetpack/modules/notes.php	0/98 (0.00%)	0.00%	-1 💚

Full summary · PHP report · JS report