Adds 8.18 backport option to mergify

[jmikell821]

Adds 8.18 backport option to mergify.

[github-actions]

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[nastasha-solomon]

This, I'm not 100% sure about since we'll no longer have a main branch to create PRs from. @lcawl should this be 8.x instead? If so, do we need to update all other instances in this file that have the base branch set to main?

[nastasha-solomon]

Nvm, this is probably not an issue!

[lcawl]

I remember a slack discussion that said this base clause was removed from the file in the main branch since mergify was only looking at that branch... which I found to be a very confusing discussion.

But i guess worst case we keep our eyes open for the first PR that starts in 8.x and see if it's successfully backported. If not, we might need to remove or change this base clause.