Mark 1.6 under critical support

[captn3m0]

What are you changing in this pull request and why?

• Changing 1.6 from Active -> Critical support as of 1.7 GA.
• This is incorrectly marked as active, hence the change.

This is based on the policy from the same page (https://docs.getdbt.com/docs/dbt-versions/core#how-dbt-core-uses-semantic-versioning) :

Newer minor versions transition the previous minor version into "Critical Support" with limited "security" releases for critical security and installation fixes.

Checklist

• Review the Content style guide and About versioning so my content adheres to these guidelines.

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[mirnawong1]

hey @captn3m0 ! thanks for opening this pr and great contribution! you're absolutely right and this is a great first-time contribution. I'll get this merged, looking forward to your future suggestions!