fix: leak fixing retain cycle of GutenbergCoverUploadProcessor

[kean]

This PR is to merge #25442. I followed the steps to run an external contributor PR build, but nothing worked. Merging it separately.

[dangermattic]

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[sonarqubecloud]

Quality Gate passed

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	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	31787
	Version	PR #25455
	Bundle ID	org.wordpress.alpha
	Commit	161716b
	Installation URL	0t2r888ulpg28

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

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	31787
	Version	PR #25455
	Bundle ID	com.jetpack.alpha
	Commit	161716b
	Installation URL	48o3ae7pbitv0

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