[CHORE] Update jsonschema library to v6

- Update the `github.com/santhosh-tekuri/jsonschema` library from v5 to
v6.
- Update the migration loader function to a type, which is now required
in V6.
- `github.com/santhosh-tekuri/jsonschema/v6` was already used by gof3,
so removing the v5 library and using the v6 library reduces the binary
size of Forgejo.
  - Before: 95912040 bytes
  - After: 95706152 bytes

modules/migration/schemas_static.go

@@ -6,10 +6,18 @@
 package migration
 
 import (
-	"io"
 	"path"
+
+	"github.com/santhosh-tekuri/jsonschema/v6"
 )
 
-func openSchema(filename string) (io.ReadCloser, error) {
-	return Assets.Open(path.Base(filename))
+type SchemaLoader struct{}
+
+func (*SchemaLoader) Load(filename string) (any, error) {
+	f, err := Assets.Open(path.Base(filename))
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return jsonschema.UnmarshalJSON(f)
 }