Add note about updating .xcode-version in Buildkite pipelines

[mokagio]

See conversation at https://github.com/wordpress-mobile/WordPress-iOS/pull/19793/files#r1052892161

To test

This change does not affect either the codebase or the automation. Green CI will suffice.

Regression Notes

1. Potential unintended areas of impact – N.A.
2. What I did to test those areas of impact (or what existing automated tests I relied on) – N.A.
3. What automated tests I added (or what prevented me from doing so) – N.A.

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• I have completed the Regression Notes.
• I have considered adding unit tests for my changes. N.A.
• I have considered adding accessibility improvements for my changes. N.A.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary. N.A.

[wpmobilebot]

You can test the changes in Jetpack from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19834-a5cffbf on your iPhone

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

[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19834-a5cffbf on your iPhone

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

[mokagio]

.xcode-version is the recommended name for the file