[Tooling] Use bot for release management git operations

[iangmaia]

References

PR adding Simplenote Android to ReleasesV2 already using the use-bot-for-git script: Automattic/simplenote-android#1688
See paaHJt-6EP-p2 for more details.

What it does

This PR updates the release pipelines to use the use-bot-for-git script to use a bot account for git operations, avoiding the need for SSH deploy keys with write permission.

Note: this PR also upgrades Xcode to 16.1-rc.1.

How to test

We'll fully run the release pipelines in the next release cycle once this is merged.

To test it, I've commented out a few side effects from start_code_freeze on b6124db and ran it directly on Buildkite (setting the env var PIPELINE=release-pipelines/start-code-freeze.yml) with this resulting build.

[dangermattic]

	1 Warning
⚠️	PR is not assigned to a milestone.

Generated by 🚫 Danger

[wpmobilebot]

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

	App Name	Simplenote Prototype Build
	Build Number	pr1666-62607e9-0192c3d7-b413-403d-91bb-e78466bac723
	Version	4.54
	Bundle ID	com.codality.NotationalFlow.Alpha
	Commit	62607e9
	App Center Build	Simplenote - Installable Builds #390

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

[mokagio]

Similar to wordpress-mobile/WordPress-iOS#23697 👍

I'll apply the suggestion to remove the shebang flags from WordPress-iOS when I'll work on the final bit of release-in-CI automation in the next few days.