Fix WKWebView retain cycle in Reader

[kean]

Fixes #

To test:

• I checked that ReaderWebViewMessageHandler gets called and ReaderWebView deinit also does

Regression Notes

1. Potential unintended areas of impact

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

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

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]

	2 Warnings
⚠️	View files have been modified, but no screenshot or video is included in the pull request. Consider adding some for clarity.
⚠️	This PR is assigned to the milestone 25.7 ❄️. 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.

	1 Message
📖	This PR contains changes to RELEASE-NOTES.txt. Note that these changes won't affect the final version of the release notes as this version is in code freeze. Please, get in touch with a release manager if you want to update the final release notes.

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📲 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	pr24028-035b0de
	Version	25.7
	Bundle ID	org.wordpress.alpha
	Commit	035b0de
	App Center Build	WPiOS - One-Offs #11422

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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	pr24028-035b0de
	Version	25.7
	Bundle ID	com.jetpack.alpha
	Commit	035b0de
	App Center Build	jetpack-installable-builds #10455

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