version: Bump version in main to 0.0.23

This reflects the v0.0.23 preview beta we made earlier this week,
which was in a branch off of main.

Possibly we should do this sort of bump immediately after making
a preview beta.  Anyway, doing it now before making the *next* beta.

pubspec.yaml

@@ -8,7 +8,7 @@ description: A Zulip client for Android and iOS
 publish_to: 'none'
 
 # Keep the last two numbers equal; see docs/release.md.
-version: 0.0.22+22
+version: 0.0.23+23
 
 environment:
   # We use a recent version of Flutter from its main channel, and