Update supported go versions to 1.20 and 1.21
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 8b660bb..d9696ad 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -83,8 +83,8 @@
     strategy:
       matrix:
         go:
-          - '1.19'
           - '1.20'
+          - '1.21'
       fail-fast: false
     steps:
       - uses: actions/checkout@v3
@@ -128,7 +128,7 @@
         run: make -C test/go precross
 
       - name: Upload go precross artifacts
-        if: matrix.go == '1.20'
+        if: matrix.go == '1.21'
         uses: actions/upload-artifact@v3
         with:
           name: go-precross