ci(golangci-lint): update configuration and fix lint issues (#4247)

* ci: update golangci-lint configuration and lint settings

- Update golangci-lint to version 2
- Enable new linters and adjust existing ones
- Update lint settings across multiple test files
- Remove unused struct and variable checks
- Add new lint exclusions for generated code and specific directories

Signed-off-by: Flc <four_leaf_clover@foxmail.com>

* ci(github): update golangci-lint-action to v8 and lint version to v2.3.4

Signed-off-by: Flc <four_leaf_clover@foxmail.com>

* ci: downgrade golangci-lint to v2.1.6

Signed-off-by: Flc <four_leaf_clover@foxmail.com>

* ci(golangci): add gofumpt linter and fix related issues- Added gofumpt linter to .golangci.yml

Signed-off-by: Flc <four_leaf_clover@foxmail.com>

* test: ignore testifylint and gofumpt lints in specific test cases

Signed-off-by: Flc <four_leaf_clover@foxmail.com>

* build(deps): remove golang.org/x/lint

- Remove golang.org/x/lint package from go.mod
- Update related dependencies in go.sum

Signed-off-by: flc1125 <four_leaf_clover@foxmail.com>

* build(deps): downgrade golang.org/x/mod and golang.org/x/tools

- Downgrade golang.org/x/mod from v0.24.0 to v0.18.0
- Downgrade golang.org/x/tools from v0.33.0 to v.22.0

These changes are made to address compatibility issues with the current project setup.

Signed-off-by: flc1125 <four_leaf_clover@foxmail.com>

---------

Signed-off-by: Flc <four_leaf_clover@foxmail.com>
Signed-off-by: flc1125 <four_leaf_clover@foxmail.com>

routes_test.go

@@ -484,7 +484,7 @@ func TestRouterMiddlewareAndStatic(t *testing.T) {
 	assert.Contains(t, w.Body.String(), "package gin")
 	// Content-Type='text/plain; charset=utf-8' when go version <= 1.16,
 	// else, Content-Type='text/x-go; charset=utf-8'
-	assert.NotEqual(t, "", w.Header().Get("Content-Type"))
+	assert.NotEmpty(t, w.Header().Get("Content-Type"))
 	assert.NotEqual(t, "Mon, 02 Jan 2006 15:04:05 MST", w.Header().Get("Last-Modified"))
 	assert.Equal(t, "Mon, 02 Jan 2006 15:04:05 MST", w.Header().Get("Expires"))
 	assert.Equal(t, "Gin Framework", w.Header().Get("x-GIN"))
@@ -764,7 +764,7 @@ func TestRouteContextHoldsFullPath(t *testing.T) {
 	// Test not found
 	router.Use(func(c *Context) {
 		// For not found routes full path is empty
-		assert.Equal(t, "", c.FullPath())
+		assert.Empty(t, c.FullPath())
 	})
 
 	w := PerformRequest(router, http.MethodGet, "/not-found")