Go 1.18 is out! So we should run tests on it.
Additionally, gqlgen had some issues with it (see 99designs/gqlgen#1961
and golang/go#45584) so I updated that too, which updated some other
things.
Finally, latest gqlgen requires 1.16+, and it's time for us to do the
same anyway, so we can use `embed` and other newer goodies. So I
dropped running tests for 1.14 and 1.15, and bumped the module language
version.
Test plan: make check
Ben Kraft