Initial update to 6.0.0-dev0: remove support of ansible-core<2.16.0 and k8s inventory plugin (#867)

SUMMARY
This is initial to prepare the main branch to version 6.0.0 (6.0.0-dev0 in galaxy.yml) and includes following braking changes:

removed support of ansible-core<2.16.0 as 2.15 reach EOL in Nov 2024;
removed the k8s inventory plugin that was deprecated in release 3.0.0.

ISSUE TYPE

Feature Pull Request

COMPONENT NAME

Documentation
galaxy.yml
inventory/k8s.py

ADDITIONAL INFORMATION
The initial version of this PR doesn't remove tests/sanity/ignore-2.14.txt and tests/sanity/ignore-2.15.txt, and CI part will require removing version 2.15 from the matrix in https://github.com/ansible-network/github_actions, so, we have external dependency here.

Reviewed-by: Bianca Henderson <beeankha@gmail.com>
Reviewed-by: Yuriy Novostavskiy
Reviewed-by: Mike Graves <mgraves@redhat.com>

docs/kubernetes.core.k8s_drain_module.rst

@@ -595,7 +595,8 @@ Examples
       kubernetes.core.k8s_drain:
         state: drain
         name: foo
-        force: yes
+        delete_options:
+          force: yes
 
     - name: Drain node "foo", but abort if there are pods not managed by a ReplicationController, Job, or DaemonSet, and use a grace period of 15 minutes.
       kubernetes.core.k8s_drain: