Fix #9970: Add security policy to project

[aranke]

resolves #9970

Problem

dbt-core doesn't have a SECURITY.md file

Solution

Add a SECURITY.md file with link to https://docs.getdbt.com/docs/dbt-versions/core

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.18%. Comparing base (8fe7d65) to head (68d525d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10136      +/-   ##
==========================================
- Coverage   88.25%   88.18%   -0.07%     
==========================================
  Files         181      181              
  Lines       22805    22805              
==========================================
- Hits        20126    20111      -15     
- Misses       2679     2694      +15     

Flag	Coverage Δ
integration	85.59% <ø> (-0.14%)	⬇️
unit	62.69% <ø> (ø)

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