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
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}