Relase 1.0: decide on minimum supported smallstep versions

[maxhoesel]

For the first major release of this collection, we should settle on both a minimum step-cli and a minimum step-ca version to support.

The step-cli version will be supported across the collection in both modules and roles, while the step-ca version will mainly be important for the step-ca role.

To-Do:

• Select a minimum version
• Update the CI config (Use Pytest for testing, enable step version tests #342 adds the ability to test older versions of the step tools)
• Update the module version detection helper
• Update the README with a table