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>

gin.go

@@ -18,7 +18,6 @@ import (
 	"github.com/gin-gonic/gin/internal/bytesconv"
 	filesystem "github.com/gin-gonic/gin/internal/fs"
 	"github.com/gin-gonic/gin/render"
-
 	"github.com/quic-go/quic-go/http3"
 	"golang.org/x/net/http2"
 	"golang.org/x/net/http2/h2c"
@@ -216,7 +215,7 @@ func New(opts ...OptionFunc) *Engine {
 		trustedProxies:         []string{"0.0.0.0/0", "::/0"},
 		trustedCIDRs:           defaultTrustedCIDRs,
 	}
-	engine.RouterGroup.engine = engine
+	engine.engine = engine
 	engine.pool.New = func() any {
 		return engine.allocateContext(engine.maxParams)
 	}