Update tested Go versions (and deps) (#179)

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

docs/CHANGELOG.md

@@ -22,6 +22,8 @@ When releasing a new version:
 
 ### Breaking changes:
 
+- genqlient now requires Go 1.16 or higher.
+
 ### New features:
 
 ### Bug fixes: