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.

modules/packages/rubygems/metadata.go

@@ -14,7 +14,7 @@ import (
 
 	"code.gitea.io/gitea/modules/validation"
 
-	"gopkg.in/yaml.v2"
+	"gopkg.in/yaml.v3"
 )
 
 var (
@@ -120,7 +120,7 @@ func (r requirement) AsVersionRequirement() []VersionRequirement {
 		if !ok {
 			continue
 		}
-		vm, ok := req[1].(map[interface{}]interface{})
+		vm, ok := req[1].(map[string]interface{})
 		if !ok {
 			continue
 		}