ci: remove Go 1.18 and add Go 1.21.

Author: ghosind

.github/workflows/coverage.yaml

@@ -12,7 +12,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v4
       with:
-        go-version: "1.20"
+        go-version: "1.21"
 
     - name: Test
       run: go test -v ./... -coverprofile=coverage.out

.github/workflows/test.yaml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        go_version: ["1.18", "1.19", "1.20"]
+        go_version: ["1.19", "1.20", "1.21"]
         os: [ubuntu-latest, windows-latest, macOS-latest]
 
     steps: