Major version bump to 12.0.0

Significant, breaking changes:

- changes the interface to the `Template` class, including removal of
  the `drop_values` argument for all template types
- removes all `Renderer` classes and moves them to methods of subclasses
  of the `Template` class

notifications_utils/version.py

@@ -1 +1 @@
-__version__ = '11.1.0'
+__version__ = '12.0.0'