Start testing on Go 1.21, stop testing on Go 1.18.

Author: bojanz

.github/workflows/build.yml

@@ -7,7 +7,7 @@ jobs:
       - name: Install Go
         uses: actions/setup-go@v4
         with:
-          go-version: 1.20.x
+          go-version: 1.21.x
 
       - name: Install tools
         run: |
@@ -36,7 +36,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go: ['1.18', '1.19', '1.20']
+        go: ['1.19', '1.20', '1.21']
     name: test @ Go ${{ matrix.go }}
     runs-on: ubuntu-latest
     steps: