Prepare 5.0.0 (#1123)

* Bump version to 5.0.0-a1.

* Drop support for ansible-core 2.15 and 2.16.

* Remove Python 2 and early Python 3 compatibility.

galaxy.yml

@@ -7,7 +7,7 @@
 
 namespace: community
 name: docker
-version: 4.8.1
+version: 5.0.0-a1
 readme: README.md
 authors:
   - Ansible Docker Working Group
@@ -15,7 +15,6 @@ description: Modules and plugins for working with Docker
 license:
   - GPL-3.0-or-later
   - Apache-2.0
-  - MIT
 # license_file: COPYING
 tags:
   - docker