Send email notification when PAT is revoked

What does this MR do and why?

Currently, revocation of a PAT happens silently. This patch adds an email notification to be triggered when a PAT is revoked.

Relevant discussion: #371911 (closed)

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a new personal access token.
2. Revoke the access token by:
   1. Rails Console:

      user = User.find_by_username "<username>"
      token = PersonalAccessToken.find(<id>)
      token.revoked = false
      group = user.groups[0]
      service = PersonalAccessTokens::RevokeService.new(current_user = user, token: token, group: group)
      service.execute

   2. PAT API: Reference
3. Check for notification email

Note: The token id can be found by running user.personal_access_token_ids in Rails console

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.