use D000 for BehaviorChangeEvent

[MichelleArk]

resolves #N/A

Updating BehaviorChangeEvent.code to D000 to avoid duplication across dbt-core/dbt-common

Confirmed no usage of D000 in dbt-core: https://github.com/dbt-labs/dbt-core/blob/1fcce443ba82ba6bd897f6135ae6bfda9d2367d/core/dbt/events/types.py#L550

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.87%. Comparing base (9d509b5) to head (5cf6d81).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #298   +/-   ##
=======================================
  Coverage   71.87%   71.87%           
=======================================
  Files          52       52           
  Lines        3644     3644           
=======================================
  Hits         2619     2619           
  Misses       1025     1025           

Flag	Coverage Δ
unit	71.87% <100.00%> (ø)

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