Support py3.12 and drop py3.8

As per the current release tested runtime, we test
from python 3.9 till python 3.12 so updating the same
in python classifier in setup.cfg

Also, dropping the python3.8 support
- https://lists.openstack.org/archives/list/openstack-discuss@lists.openstack.org/thread/FOWV4UQZTH4DPDA67QDEROAESYU5Z3LE/

Change-Id: Ia27257e0ab90979c59c479c2ddf946e926c1aed3

releasenotes/notes/drop-python38-support-07c1fab194bad54a.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 3.8 support has been dropped. Last release of
+    manila-tempest-plugin to support python 3.8 is 2.5.0.
+    The minimum version of Python now supported is Python 3.9.

releasenotes/notes/remove-py-36-37-5d61c3dcf682e0df.yaml

@@ -1,5 +0,0 @@
----
-upgrade:
-  - |
-    Support for Python 3.6 and 3.7 has been removed. Now the minimum version
-    supported is 3.8 .

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/manila/latest/
-python_requires = >=3.8
+python_requires = >=3.9
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,7 +15,6 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10
     Programming Language :: Python :: 3.11