Use next stable release section in CHANGELOG.md instead of dev releases.

Having a section for each dev release causes a difference between the master
and dev branch. That means there's always a merge conflict when doing dev
releases, and that doing a dev release requires merging the dev CHANGELOG.md
back to master. We can avoid that churn by instead having a section for the
next stable release on the master branch.

This change means the master branch will no longer contain the history of
the dev releases leading up to the next stable release. However, we merge
all of those entries together anyway when doing a stable release, and the
changes in each dev release can be consulted by checking out the appropriate
tag.

Change-Id: If34651be4ccadb74fcce4a0004ab109bb003dd01
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106346
Reviewed-by: Alexander Thomas <athom@google.com>

CHANGELOG.md

@@ -1,6 +1,6 @@
-## 2.4.1-dev.XX.0
+## Next release
 (Add new changes here, and they will be copied to the change section for the
- next dev version)
+ next release)
 
 ### Language