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: I088d48efa1de74a24a365990a9396514a3a76ad2

releasenotes/notes/drop-python-3-6-and-3-7-fa2dda5d6be0cad6.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 3.6 & 3.7 support has been dropped. The minimum version of Python now
+    supported is Python 3.8.

tox.ini

@@ -49,10 +49,6 @@ commands = sphinx-build -a -E -W -j auto -d releasenotes/build/doctrees -b html
 setenv =
   OS_TEST_PATH = ./cinder/tests/functional
 
-[testenv:functional-py36]
-setenv =
-  {[testenv:functional]setenv}
-
 [testenv:functional-py38]
 setenv =
   {[testenv:functional]setenv}