Prepare release notes for v1.3.0 (#333)

Author: llucax

RELEASE_NOTES.md

@@ -1,13 +1,5 @@
 # Frequenz channels Release Notes
 
-## Summary
-
-<!-- Here goes a general summary of what this release is about -->
-
-## Upgrading
-
-<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
-
 ## New Features
 
 - There is a new `Receiver.triggered` method that can be used instead of `selected_from`:
@@ -21,7 +13,3 @@
   ```
 
 * `Receiver.filter()` can now properly handle `TypeGuard`s. The resulting receiver will now have the narrowed type when a `TypeGuard` is used.
-
-## Bug Fixes
-
-<!-- Here goes notable bug fixes that are worth a special mention or explanation -->