(#185) Update ansible-core versions supported/tested

[Windos]

Description Of Changes

• Set collection minimum ansible-core version to 2.18, the current lowest version that is still supported
• Update CI testing to test 2.18-2.20
• Build collection on 2.20 of ansible-core
• Update readme

Motivation and Context

We need to keep in-step with Ansible's supported versions of ansible-core and ensure we're testing appropriately.

Testing

Running tests in CI from the PR

Operating Systems Testing

N/A

Change Types Made

• Bug fix (non-breaking change).
• Feature / Enhancement (non-breaking change).
• Breaking change (fix or feature that could cause existing functionality to change).
• Documentation changes.
• PowerShell code changes.

Change Checklist

• Requires a change to the documentation.
• Documentation has been updated.
• Tests to cover my changes, have been added.
• All new and existing tests passed?
• PowerShell code changes: PowerShell v3 compatibility checked?

Related Issue

Fixes #185