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>

models/repo.go

@@ -9,8 +9,6 @@ import (
 	"strconv"
 	"strings"
 
-	_ "image/jpeg" // Needed for jpeg support
-
 	"code.gitea.io/gitea/models/db"
 	issues_model "code.gitea.io/gitea/models/issues"
 	repo_model "code.gitea.io/gitea/models/repo"
@@ -18,6 +16,8 @@ import (
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/log"
 
+	_ "image/jpeg" // Needed for jpeg support
+
 	"xorm.io/builder"
 )