chore: update github workflow to use Go1.21

Author: jszwec

.github/workflows/go.yml

@@ -11,7 +11,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: ["1.20"]
+        go-version: ["1.21"]
         os: [ubuntu-latest, macos-latest, windows-latest]
 
     runs-on: ${{ matrix.os }}
@@ -35,7 +35,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go-version: [1.18, 1.19]
+        go-version: ["1.19", "1.20"]
     steps:
     - uses: actions/checkout@v2
       with: