Merge 21.6 code freeze into trunk

[mokagio]

• Update internal Pods to stable version
• New version header in RELEASE-NOTE.txt
• Localizable.strings updated
• release_notes.txt updated with notes from RELEASE-NOTE.txt for current version
• Version update in .xcconfig

[wpmobilebot]

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

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• Then installing the build number pr19964-cea2210 on your iPhone

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[wpmobilebot]

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

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• Then installing the build number pr19964-cea2210 on your iPhone

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[spencertransier]

Approved minus the merge conflict with Podfile.lock. I switched to that branch and ran bundle exec pod install, but it didn't update the checksum. What's the correct way to resolve that conflict?

[mokagio]

What's the correct way to resolve that conflict?

To resolve the conflict, we need to attempt to merge the base branch into the PR branch. This operation will result in the conflict locally. We can then manually address it and commit the merge.

You already know this, but for reference: Since this is a release/* branch, we don't want to merge trunk into it, or all the other changes that landed on trunk since cutting release/21.6 will end up in the next beta. Instead, we create a new branch from release/21.6 and do the merge, resolution, and PR there. See #19973.

I switched to that branch and ran bundle exec pod install, but it didn't update the checksum.

There are instances in which CI raises inconsistency issues in the Podfile.lock. Running bundle exec pod install on the branch that raised the error usually results in the checksum updating, because the most usual cause of the issue in the first place is a change in Podfile that was not followed by running that command. However, notice that this kind of CI error is not a merge conflict.

[mokagio]

Closing in favor of #19973.