Replace custom Go formatter with golangci-lint fmt (#37194)

Use `golangci-lint fmt` to format code, replacing the previous custom
formatter tool. https://github.com/daixiang0/gci is used to order the
imports.

`make fmt` performs ~13% faster while consuming ~57% less cpu while
formatting for me.

`GOFUMPT_PACKAGE` is gone because it's using the builtin package from
golangci-lint.

Co-authored-by: Claude (claude-opus-4-6) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/avatar/avatar.go

@@ -11,15 +11,14 @@ import (
 	"image/color"
 	"image/png"
 
-	_ "image/gif"  // for processing gif images
-	_ "image/jpeg" // for processing jpeg images
-
 	"code.gitea.io/gitea/modules/avatar/identicon"
 	"code.gitea.io/gitea/modules/setting"
 
-	"golang.org/x/image/draw"
-
 	_ "golang.org/x/image/webp" // for processing webp images
+	_ "image/gif"               // for processing gif images
+	_ "image/jpeg"              // for processing jpeg images
+
+	"golang.org/x/image/draw"
 )
 
 // DefaultAvatarSize is the target CSS pixel size for avatar generation. It is