[Tooling] Remove Backmerge PR creation from the publish_release lane

[iangmaia]

Reference: p1734366091645069-slack-CC7L49W13

After a discussion with @oguzkocer, we decided to remove the automatic creation of backmerge PRs from the publish_release lane. The main points:

• Backmerge PRs can be problematic when repositories use a squash-merge strategy for Pull Requests
• At this point, release branch changes should already be in the default branch
• For exceptional cases (e.g., app store rejections requiring last-minute changes), teams can create backmerge PRs manually as needed

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 25.6 ❄️. 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.

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📲 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
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	Build Number	pr23900-ce2b3a1
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	Commit	ce2b3a1
	App Center Build	jetpack-installable-builds #10225

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