More Mergify improvements

[michaelsproul]

We've adopted some new Mergify patterns in:

• Fix branch/tag names in mergify config #6842
• add new features to mergify config #6597

There are still a few outstanding issues.

• I got the label name wrong for ready-for-merge. The config is incorrectly using ready-to-merge. We could rename the label, or update the config (which we probably want to do anyway).
• We should ideally remove approval on trivial PRs if the trivial label is removed.
• We should not let the bot approve trivial PRs that have conflicts.
• We should not let the bot queue PRs that are labelled do-not-merge (this is orthogonal to the recent changes, but might be nice to have).

[michaelsproul]

cc @jxs