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

.github/workflows/go.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: [ '1.16', '1.17', '1.18', '1.19' ]
+        go: [ '1.18', '1.19', '1.20' ]
 
     steps:
     - name: Set up Go
@@ -42,4 +42,4 @@ jobs:
     - name: Run lint
       uses: golangci/golangci-lint-action@v2
       with:
-        version: v1.48   # should match internal/lint/go.mod
+        version: v1.52.2   # should match internal/lint/go.mod