ci: Test using Go 1.21

Author: TwiN

.github/workflows/test.yml

@@ -16,7 +16,7 @@ jobs:
     steps:
       - uses: actions/setup-go@v5
         with:
-          go-version: 1.20
+          go-version: 1.21
       - uses: actions/checkout@v4
       - run: go test ./... -race -coverprofile=coverage.txt -covermode=atomic
       - name: Codecov