docs: update SECURITY.md with reporting guidance

[tabathad]

Clarifies the preferred vulnerability disclosure process by directing users to report via GitHub and linking to the FINOS security vulnerabilities policy.

Closes 1018

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: tabathad / name: Tabatha DiDomenico (bd5f41b)
• ✅ login: jescalada / name: Juan Escalada (44c763f)

[netlify]

✅ Deploy Preview for endearing-brigadeiros-63f9d0 canceled.

Name	Link
🔨 Latest commit	44c763f
🔍 Latest deploy log	https://app.netlify.com/projects/endearing-brigadeiros-63f9d0/deploys/6884481eec955c0008824ae9

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.40%. Comparing base (9523385) to head (44c763f).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1117   +/-   ##
=======================================
  Coverage   77.40%   77.40%           
=======================================
  Files          56       56           
  Lines        2288     2288           
  Branches      258      258           
=======================================
  Hits         1771     1771           
  Misses        487      487           
  Partials       30       30           

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[jescalada]

Looks good to me! 🚀