Update tested Go versions (and deps) (Khan#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

.gitattributes

@@ -0,0 +1 @@
+go.sum linguist-generated

.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

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:

go.mod

@@ -1,13 +1,15 @@
 module github.com/Khan/genqlient
 
-go 1.14
+go 1.16
 
 require (
-	github.com/99designs/gqlgen v0.14.0
+	github.com/99designs/gqlgen v0.17.2
+	github.com/agnivade/levenshtein v1.1.1 // indirect
 	github.com/alexflint/go-arg v1.4.2
 	github.com/bradleyjkemp/cupaloy/v2 v2.6.0
 	github.com/stretchr/testify v1.7.0
-	github.com/vektah/gqlparser/v2 v2.3.1
-	golang.org/x/tools v0.1.7
+	github.com/vektah/gqlparser/v2 v2.4.1
+	golang.org/x/sys v0.0.0-20220319134239-a9b59b0215f8 // indirect
+	golang.org/x/tools v0.1.10
 	gopkg.in/yaml.v2 v2.4.0
 )