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: If5a3b729f155770ea6b4a088e58a9668ff90c5b1

releasenotes/notes/drop-python38-support-1a8f8b06fcc028c5.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 3.8 support has been dropped. Last release of
+    designate-tempest-plugin to support python 3.8 is 0.23.0.
+    The minimum version of Python now supported is Python 3.9.

releasenotes/notes/remove-py-36-37-d7724851f8db349d.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/designate-tempest-plugin/latest/
-python_requires = >=3.8
+python_requires = >=3.9
 classifier =
     Environment :: OpenStack
     Environment :: No Input/Output (Daemon)
@@ -18,7 +18,6 @@ classifier =
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10
     Programming Language :: Python :: 3.11