Revise security policy and reporting instructions

[willingc]

Updated the security policy to emphasize responsible disclosure and provide clear instructions for reporting vulnerabilities.

Also enabled private vulnerability reporting in the Security and quality tab above.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.01%. Comparing base (ba0992e) to head (6277d3a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #357   +/-   ##
=======================================
  Coverage   79.01%   79.01%           
=======================================
  Files          12       12           
  Lines         810      810           
  Branches      107      107           
=======================================
  Hits          640      640           
  Misses        155      155           
  Partials       15       15           

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