Drop CHANGES.rst in favor of GitHub Releases

I started to cut a new release but realized it's a bit painful that we
are currently maintaining both a `CHANGES.rst` file and also tagging
releases in GitHub releases UI.

For a busy project, maintaining a dedicated changelog makes sense... but
the reality is this project is in maintenance mode, and we the
maintainers are more likely to cut releases when they're easy/low
friction. Since GitHub very nicely has the "Auto-generate-release-notes"
button, let's just use that.

I considered copy/pasting the results from that to `CHANGES.rst`, but
even that feels like a waste of time.

setup.cfg

@@ -11,7 +11,7 @@ long_description_content_type = text/x-rst
 keywords = flask, debug, toolbar
 url = https://github.com/pallets-eco/flask-debugtoolbar
 project_urls =
-    Changelog = https://github.com/pallets-eco/flask-debugtoolbar/blob/master/CHANGES.rst
+    Changelog = https://github.com/pallets-eco/flask-debugtoolbar/releases
     Documentation = https://flask-debugtoolbar.readthedocs.io/
 classifiers =
     Development Status :: 4 - Beta