Bump minimum Go version to 1.18 (#262)

This means we can use generics and various other things. I didn't use
any of them yet, this is just bumping the numbers. I added tests for
1.20, and fixed one small bug, I think caused by `go/packages` changes
therein. And I bumped the golangci-lint version too while I was in the
area.

Fixes #256, fixes #257.

I have:
- [x] Written a clear PR title and description (above)
- [x] Signed the [Khan Academy CLA](https://www.khanacademy.org/r/cla)
- [x] Added tests covering my changes, if applicable
- [x] Included a link to the issue fixed, if applicable
- [x] Included documentation, for new features
- [x] Added an entry to the changelog

docs/CHANGELOG.md

@@ -22,6 +22,8 @@ When releasing a new version:
 
 ### Breaking changes:
 
+- genqlient now requires Go 1.18 or higher.
+
 ### New features:
 
 - You can now bind all types from a package in `genqlient.yaml` using the new `package_bindings` option.