automation: use gp3 volume type
This is a newer volume type. It offers a better baseline of performance
by default and is ~10% cheaper. It doesn't offer burst credits. But
its performance is guaranteed, unlike gp2, which had wonky performance
behavior.
Differential Revision: https://phab.mercurial-scm.org/D12132
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).