Add releasenotes to drop python3.6|7

As per testing runtime for Zed cycle, below patch dropped the
support for python 3.6|7 and having a releasenotes for that will
be helpful for users.

- https://review.opendev.org/c/openstack/cinder/+/839469

[1] https://governance.openstack.org/tc/reference/runtimes/zed.html

Change-Id: Ic7116b0ad50e5eb81f487f60746981b70356f494

tox.ini

@@ -38,9 +38,6 @@ norecursedirs  = .tox .venv
 [testenv:venv]
 commands = {posargs}
 
-[testenv:py36]
-basepython = python3.6
-
 [testenv:py38]
 basepython = python3.8