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>

README.md

@@ -23,7 +23,7 @@ For more information about communication, see the [Ansible communication guide](
 <!--start requires_ansible-->
 ## Ansible version compatibility
 
-This collection has been tested against following Ansible versions: **>=2.15.0**.
+This collection has been tested against following Ansible versions: **>=2.16.0**.
 
 For collections that support Ansible 2.9, please ensure you update your `network_os` to use the
 fully qualified collection name (for example, `cisco.ios.ios`).
@@ -57,11 +57,6 @@ Name | Description
 --- | ---
 kubernetes.core.k8s_config_resource_name|Generate resource name for the given resource of type ConfigMap, Secret
 
-### Inventory plugins
-Name | Description
---- | ---
-[kubernetes.core.k8s](https://github.com/ansible-collections/kubernetes.core/blob/main/docs/kubernetes.core.k8s_inventory.rst)|Kubernetes (K8s) inventory source
-
 ### Lookup plugins
 Name | Description
 --- | ---