ci(conf): add Go 1.21 to the matrix

Author: tucksaun

.github/workflows/test.yaml

@@ -14,6 +14,7 @@ jobs:
         - '1.18'
         - '1.19'
         - '1.20'
+        - '1.21'
     name: Go ${{ matrix.go }} test
     steps:
       - uses: actions/checkout@v2