chore: cdktfVersion -> cdktnVersion

[gabegorelick]

Related issue

Fixes #

Description

Update references from cdktfVersion to cdktnVersion.

Checklist

• I have updated the PR title to match CDKTN's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes

[X-Guardian]

You need to update the integration test snapshots for this change.

[gabegorelick]

I need to reassess what the scope of this change should be. I split off #228 for the user-facing, breaking change.