cmd/ursrv: Properly sort versions (fixes #6991)

cmd/ursrv/main.go

@@ -28,6 +28,7 @@ import (
 
 	"github.com/oschwald/geoip2-golang"
 
+	"github.com/syncthing/syncthing/lib/upgrade"
 	"github.com/syncthing/syncthing/lib/ur/contract"
 )
 
@@ -978,7 +979,9 @@ func (s *summary) MarshalJSON() ([]byte, error) {
 	for v := range s.versions {
 		versions = append(versions, v)
 	}
-	sort.Strings(versions)
+	sort.Slice(versions, func(a, b int) bool {
+		return upgrade.CompareVersions(versions[a], versions[b]) < 0
+	})
 
 	var filtered []string
 	for _, v := range versions {