Remove afterEvaluate together with support for older versions of Ktlint plugin

[tasomaniac]

afterEvaluate usage is not good since it requires the clients to use afterEvaluate in certain conditions.

This was required initially because ktlint plugin itself was using it. That is fixed quite a while ago.

Solution:

• Remove afterEvaluate
• Remove older version support for ktlint
• Update the docs to indicate that we support version 6 and above

Fixes #156

[tasomaniac]

Yes, just fixed it. 🙌