Replace extra decorator with property and lazy initialization so that it is backward compatible.

betterproto/templates/template.py

@@ -1,6 +1,7 @@
 # Generated by the protocol buffer compiler.  DO NOT EDIT!
 # sources: {{ ', '.join(description.files) }}
 # plugin: python-betterproto
+from dataclasses import dataclass
 {% if description.datetime_imports %}
 from datetime import {% for i in description.datetime_imports %}{{ i }}{% if not loop.last %}, {% endif %}{% endfor %}
 
@@ -37,7 +38,7 @@ class {{ enum.py_name }}(betterproto.Enum):
 {% endfor %}
 {% endif %}
 {% for message in description.messages %}
-@betterproto.protoclass
+@dataclass
 class {{ message.py_name }}(betterproto.Message):
     {% if message.comment %}
 {{ message.comment }}