Send email notification when PAT is rotated

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What does this MR do and why?

Currently, rotation of a PAT happens silently. This MR adds an email notification to be triggered when a PAT is successfully rotated.

Fix #557529 (closed)

References

Related #337591 (closed) #371911 (closed) !81340 (merged) !98911 (merged)

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Create a new personal access token.
2. Rotate the personal access token (using the PAT API or the PAT UI).
3. Check for notification email -> open locally http://localhost:3000/rails/mailers/

MR acceptance checklist

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