[8.x] [main] Update .mergify.yml

[jmikell821]

Updates main branch. Adds 8.x mergify for v.8.18.0 branch; updates previous 8.x to target 8.17.

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @jmikell821? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[nastasha-solomon]

This looks right, but I'd still defer to @lcawl for the final approval.