Adjust PR checklist formatter discussion (#8924)
Adds a link to the specific autoformatter command we expect contributors to run, since it's not currently very obvious in our docs.
Also removes an obsolete reference to being different from flutter/flutter in autoformatting Dart, now that flutter/flutter does use the formatter.
Update the rules for the post-merge labeler (#379)
The existing pattern isn't matching; this replaces it with more explicit rules that are more similar to what flutter/plugins is successfully using.