[Tooling] Fix `update_appstore_strings` backmerge PR creation

[iangmaia]

This PR should fix the issue highlighted by @oguzkocer on this change, where update_appstore_strings wouldn't properly create the PR to merge the release_notes/{version} branch to release/{version}.

iOS counterpart: https://github.com/wordpress-mobile/WordPress-iOS/pull/23758

---

To Test:

This will be fully tested during the regular release cycle.

It is possible to simulate a similar scenario following the steps below:

1. Create a dummy local branch
2. Create and push a dummy release branch with a number far in the future (e.g. release/500.0)
3. Go back to the initial local dummy branch
4. Run the lane with bundle exec fastlane update_appstore_strings version:"500.0"
5. A PR from the local branch (which, in a real world scenario, will be the release note branch) should be created.

[wpmobilebot]

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

	App Name	WordPress
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr21397-a0f00a1
	Commit	a0f00a19180ef5424280c44b6c9a239a3eef3bb1
	Direct Download	wordpress-prototype-build-pr21397-a0f00a1.apk

Note: Google Login is not supported on these builds.

[wpmobilebot]

📲 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
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr21397-a0f00a1
	Commit	a0f00a19180ef5424280c44b6c9a239a3eef3bb1
	Direct Download	jetpack-prototype-build-pr21397-a0f00a1.apk

Note: Google Login is not supported on these builds.

[dangermattic]

	1 Warning
:warning:	This PR is assigned to the milestone 25.5. 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.

Generated by :no_entry_sign: Danger

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud