Run tests on Go 1.18+

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.16, 1.17]
+        go-version: [1.18, 1.23]
         platform: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.platform }}
 
@@ -30,13 +30,11 @@ jobs:
       run: diff -u <(echo -n) <(go fmt $(go list ./...))
 
     - name: Check Go modules
-      if: matrix.go-version == '1.17'
       run: |
         go mod tidy
         git diff --exit-code
 
     - name: Check formatting
-      if: matrix.go-version == '1.17'
       run: diff -u <(echo -n) <(gofmt -d .)
 
     - name: Run unit tests