chore: update linters and go-version in GitHub actions workflow

- Update the `go-version` to `"^1.21"` in the `.github/workflows/go.yml` file
- Remove the `deadcode` linter and add the `structcheck` and `varcheck` linters in the `.golangci.yml` file

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

.github/workflows/go.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Setup go
         uses: actions/setup-go@v4
         with:
-          go-version: "^1.18"
+          go-version: "^1.21"
       - name: Checkout repository
         uses: actions/checkout@v4
       - name: Setup golangci-lint

.golangci.yml

@@ -4,7 +4,6 @@ linters:
   fast: false
   enable:
     - bodyclose
-    - deadcode
     - depguard
     - dogsled
     - dupl
@@ -29,13 +28,11 @@ linters:
     - nolintlint
     - rowserrcheck
     - staticcheck
-    - structcheck
     - stylecheck
     - typecheck
     - unconvert
     - unparam
     - unused
-    - varcheck
     - whitespace
     - gofumpt