add go1.21 to CI (#522)

Author: techknowlogick

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [ 1.15.x, 1.16.x, 1.17.x, 1.18.x, 1.19.x, 1.20.x ]
+        go-version: [ 1.15.x, 1.16.x, 1.17.x, 1.18.x, 1.19.x, 1.20.x, 1.21.x ]
         os: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.os }}
     steps: