Replace yaml.v2 with yaml.v3 (#21832)

I don't see why we have to use two versions of yaml. The difference
between the two versions has nothing to do with our usage.

services/migrations/dump.go

@@ -26,7 +26,7 @@ import (
 	"code.gitea.io/gitea/modules/structs"
 
 	"github.com/google/uuid"
-	"gopkg.in/yaml.v2"
+	"gopkg.in/yaml.v3"
 )
 
 var _ base.Uploader = &RepositoryDumper{}