Adds 8.15 branch to backport and mergify configs

[karenzone]

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[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @karenzone? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[kilfoyle]

LGTM! 🚢
Thanks!