Migrate from Intel to Apple Silicon CI

[spencertransier]

This PR migrates Simplenote iOS from Intel to Apple Silicon CI.

The "Build and Test" step succeeded, which builds the app and runs the unit tests: https://buildkite.com/automattic/simplenote-ios-apple-silicon/builds/3. The "UI Test (Full)" step fails due to a known issue with the test accounts that are used in the UI tests: #1536 (comment)

I ran the UI tests locally on my Apple Silicon Mac and 37 out of 43 passed. The six that failed don't appear to be due to any Intel vs. Apple Silicon difference, so I don't think they should be a blocker to this migration.

Accompanying buildkite-ci PR to update the Terraform config: https://github.com/Automattic/buildkite-ci/pull/338

[wpmobilebot]

You can test the changes in simplenote-ios from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr1540-4d9326e-018cadd6-2a17-4a5f-a7f8-7a9958fce82c on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[peril-automattic]

	Warnings
⚠️	PR is not assigned to a milestone.

Generated by 🚫 dangerJS

[AliSoftware]

I'm happily surprised that there were no changes needed in the codebase to migrate SN from Xcode 14.3 to Xcode 15.1, especially given the app is not really maintained anymore those days! 🎉

[AliSoftware]

PS: CI is red because the config was apparently reverted to unclustered in Buildkite (probably via a terraform apply from an unrelated buildkite-ci PR) and the last build was thus run on Intel infra, where xcode-15.1 does not exist. So this is not surprising.

@spencertransier Once you apply https://github.com/Automattic/buildkite-ci/pull/338 (which I just approved) to deploy the changes, I'd suggest you to rebuild the last commit to validate that the changes were taken into account

• I think you can just do this using the "Rebuild" button in Buildkite… but that being said, I'm not 100% sure that this won't reuse the settings of the build it is rebuilding (instead of re-evaluating the updated pipeline settings, like the cluster to use)
• So, alternatively you might have to push an empty commit (git commit --allow-empty -m"Empty commit to retrigger CI" && git push) instead.