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

.golangci.yml

@@ -7,17 +7,14 @@ linters:
   disable-all: true
   enable:
     # golangci enables these by default.
-    - deadcode
     - errcheck
     - gofumpt  # (replaces gofmt)
     - gosimple
     - govet
     - ineffassign
     - staticcheck
-    - structcheck
     - typecheck
     - unused
-    - varcheck
     # golangci disables these by default, but we use them.
     - bodyclose
     - depguard