Use Xcode 15.0.1

[jkmassel]

Migrates the project to Xcode 15 and Apple Silicon in CI

Also includes:

• An adjustment to the UITextView subclass that injects a UIPasteboard object. This allows us to mock access to the system pasteboard, which requires user interaction in recent iOS versions
• An adjustment to string index handling – in newer versions of Xcode, there are runtime assertions about string index overflow, so we need to be a bit more careful when replacing text
• A tiny little fix for a failing test, presumably related to something about how iOS generates PNG files.

This can be reviewed by anyone, but I'd appreciate if @mokagio or @crazytonyli took a look at the project code adjustments!

Testing Details

Ensure CI passes

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• [x] Please check here if your pull request includes additional test coverage.
• [x] I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[AliSoftware]

Breadcrumbs: I reverted the attribution of the cluster to this repo in Buildkite (https://github.com/Automattic/buildkite-ci/pull/331) — aka reverted this repo to use Intel infra — while this PR is still not merged.

Once we land this PR, don't forget to redo the change in aztec-editor-ios.tf to switch it back to use the cluster again.

[mokagio]

I wanted to help here and tried to address the unit tests failures via https://github.com/wordpress-mobile/AztecEditor-iOS/pull/1382 but didn't get very far.

[mokagio]

Going to merge this despite unit tests failures, to unblock moving to the cluster in CI, which in turns unblocks adopting the latest Xcode, https://github.com/wordpress-mobile/AztecEditor-iOS/pull/1392, which, finally, will unblock https://github.com/wordpress-mobile/AztecEditor-iOS/pull/1391.

I'm still experiencing unit tests issue locally, but I want to see how/if the situation changes once this lands.