fix(oidc): ignore unknown language tag in userinfo unmarshal

Open system reported an issue where a generic OpenID provider might return language tags like "gb".
These tags are well-formed but unknown and Go returns an error for it.
We already ignored unknown tags is ui_locale arrays lik in AuthRequest.

This change ignores singular unknown tags, like used in the userinfo `locale` claim.

pkg/oidc/userinfo.go

@@ -41,7 +41,13 @@ func (u *UserInfo) MarshalJSON() ([]byte, error) {
 }
 
 func (u *UserInfo) UnmarshalJSON(data []byte) error {
-	return unmarshalJSONMulti(data, (*uiAlias)(u), &u.Claims)
+	if err := unmarshalJSONMulti(data, (*uiAlias)(u), &u.Claims); err != nil {
+		return err
+	}
+	if u.Locale != nil && u.Locale.tag.IsRoot() {
+		u.Locale = nil
+	}
+	return nil
 }
 
 type UserInfoProfile struct {