models: Replace realm.uri with realm.url.

In #23380, we are changing all occurrences of uri with url in order to
follow the latest URL standard. Previous PRs #25038 and #25045 has
replaced the occurences of uri that has no direct relation with realm.

This commit changes just the model property, which has no API
compatibility concerns.

zerver/models/realms.py

@@ -898,7 +898,7 @@ class Realm(models.Model):  # type: ignore[django-manager-missing] # django-stub
         return self.string_id
 
     @property
-    def uri(self) -> str:
+    def url(self) -> str:
         return settings.EXTERNAL_URI_SCHEME + self.host
 
     @property