Convert unowned to weak in ReaderCommentsFollowPresenter

[dvdchr]

Fixes #21730
Refs #22711

This converts the unowned property specifier for presentingViewController to weak, which addresses JETPACK-IOS-1796 and JETPACK-IOS-W96.

To test

• Launch the Jetpack app
• Navigate to any comment thread
• Tap the "Follow" button on the top right
• 🔎 Verify that you can Subscribe to post comments
• Tap the notification bell icon to open the settings sheet, and toggle the in-app notification switch.
• 🔎 Verify that you can subscribe/unsubscribe to in-app notifications
• Tap "Unfollow Conversation"
• 🔎 Verify that you can unfollow the thread successfully.

Additionally: Verify the Follow process from the Reader detail screen

Regression Notes

1. Potential unintended areas of impact
   Should be none. No logic is altered with this change.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manually tested the changes.

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:
N/A

• 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)

[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	pr22712-a8f85c9
	Version	24.3
	Bundle ID	org.wordpress.alpha
	Commit	a8f85c9
	App Center Build	WPiOS - One-Offs #8998

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 Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22712-a8f85c9
	Version	24.3
	Bundle ID	com.jetpack.alpha
	Commit	a8f85c9
	App Center Build	jetpack-installable-builds #8036

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

[wargcm]

LGTM!