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

.github/workflows/go.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: [ '1.14', '1.15', '1.16', '1.17' ]
+        go: [ '1.16', '1.17', '1.18' ]
 
     steps:
     - name: Set up Go