[Notifications P1] Update header style so header won't float

[alpavanoglu]

Refs: #22671

Description

We decided to make headers unfixed on scroll. This is achieved by setting header style to grouped

The spacings are unchanged since @osullivanchris wanted to see an updated version first before altering them.

Testing Steps

1. Install & Login Jetpack
2. Navigate to Notifications tab.
3. ✅ Verify the headers are not fixed on scroll and scroll with the content.

Regression Notes

1. Potential unintended areas of impact
   N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   N/A

3. What automated tests I added (or what prevented me from doing so)
   N/A

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
⚠️	View files have been modified, but no screenshot or video is included in the pull request. Consider adding some for clarity.

Generated by 🚫 Danger

[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	pr22680-6245912
	Version	24.3
	Bundle ID	org.wordpress.alpha
	Commit	6245912
	App Center Build	WPiOS - One-Offs #8922

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[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	pr22680-6245912
	Version	24.3
	Bundle ID	com.jetpack.alpha
	Commit	6245912
	App Center Build	jetpack-installable-builds #7952

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

[justtwago]

Tested the change and headers are scrolled along with all the notifications. 👍