refactor: reword default string for min_python

Author: vchudnov-g

google/api_core/_python_version_support.py

@@ -221,7 +221,7 @@ def min_python(date: datetime.date) -> str:
         for version, info in sorted(PYTHON_VERSION_INFO.items()):
             if info.python_start <= date < info.python_eol:
                 return f"{version[0]}.{version[1]}"
-        return "at a supported version"
+        return "at a currently supported version [https://devguide.python.org/versions]"
 
     if gapic_end < today:
         message = _flatten_message(