.github/workflows: test on Go 1.21

Run the build checks on 1.18 (the oldest supported version) and the latest two
releases supported by upstream Go: 1.20 and 1.21. Run the static checks
on Go 1.21.

.github/workflows/linux.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         # Oldest we support is 1.18, plus the latest two releases.
-        go-version: ['1.18', '1.19', '1.20']
+        go-version: ['1.18', '1.20', '1.21']
     runs-on: ubuntu-latest
 
     steps:
@@ -27,13 +27,13 @@ jobs:
       uses: actions/checkout@v3
 
     - name: Check Go modules
-      if: matrix.go-version == '1.20'
+      if: matrix.go-version == '1.21'
       run: |
         go mod tidy
         git diff --exit-code
 
     - name: Check formatting
-      if: matrix.go-version == '1.20'
+      if: matrix.go-version == '1.21'
       run: diff -u <(echo -n) <(gofmt -d .)
 
     - name: Run tests on linux

.github/workflows/staticcheck.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v2
       with:
-        go-version: '1.20'
+        go-version: '1.21'
 
     - name: Check out code
       uses: actions/checkout@v3