Prepare release notes for v0.11.3

Document protobuf 7 support so clients can install the newly released
Dispatch API alongside client-base. Keep the existing keep-alive fix
notes and remove template comments and the empty new-features section
as required by the contribution guide before tagging a release.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

RELEASE_NOTES.md

@@ -2,15 +2,14 @@
 
 ## Summary
 
-Fix HTTP/2 keep-alive, which was never actually enabled.
+This release adds support for protobuf 7 and fixes HTTP/2 keep-alive, which was
+never actually enabled.
 
 ## Upgrading
 
-<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
-
-## New Features
-
-<!-- Here goes the main new features and examples or instructions on how to use them -->
+- The protobuf requirement is now `>=5.29.2,<8`, allowing protobuf 7 while
+  retaining support for protobuf 5 and 6. This permits installation alongside
+  `frequenz-api-dispatch` 1.1.0, which requires protobuf `>=7.36.1,<9`.
 
 ## Bug Fixes