add warn_once

[colin-rogers-dbt]

resolves #

Description

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.35%. Comparing base (98ccfce) to head (678d5dd).

Files with missing lines	Patch %	Lines
dbt_common/behavior_flags.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #196      +/-   ##
==========================================
- Coverage   67.35%   67.35%   -0.01%     
==========================================
  Files          52       52              
  Lines        3370     3376       +6     
==========================================
+ Hits         2270     2274       +4     
- Misses       1100     1102       +2     

Flag	Coverage Δ
unit	67.35% <66.66%> (-0.01%)	⬇️

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