[TECH DEBT] Deprecate Docker Modules

[garethgreenaway]

Description of the tech debt to be addressed, include links and screenshots

Deprecating all Docker functionality in Salt and moving it to:
https://github.com/saltstack/saltext-docker

First Salt version with Vault no longer present in the core: 3009.0

To be removed:

tests/pytests/functional/modules/test_dockermod.py
tests/pytests/functional/states/test_docker_network.py
tests/pytests/functional/states/test_docker_container.py
tests/pytests/unit/modules/dockermod
tests/pytests/unit/states/test_docker_image.py
tests/pytests/unit/states/test_docker_volume.py
tests/integration/files/file/base/docker_non_root
tests/unit/utils/test_dockermod.py
salt/utils/dockermod
salt/modules/dockermod.py
salt/modules/dockercompose.py
salt/executors/docker.py
salt/engines/docker_events.py
salt/proxy/docker.py
salt/states/docker_image.py
salt/states/docker_container.py
salt/states/docker_network.py
salt/states/docker_volume.py
doc/ref/modules/all/salt.modules.dockermod.rst
doc/ref/modules/all/salt.modules.dockercompose.rst
doc/ref/executors/all/salt.executors.docker.rst
doc/ref/engines/all/salt.engines.docker_events.rst
doc/ref/proxy/all/salt.proxy.docker.rst
doc/ref/states/all/salt.states.docker_container.rst
doc/ref/states/all/salt.states.docker_volume.rst
doc/ref/states/all/salt.states.docker_image.rst
doc/ref/states/all/salt.states.docker_network.rst
doc/topics/tutorials/docker_sls.rst