Make SECURITY.md consistent

[evankanderson]

We're missing these across a lot of Knative repos, this is copied/improved from the 3 that existed.

[davidhadas]

/retest

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6f0b90d) 63.87% compared to head (c1c9116) 64.83%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2089      +/-   ##
==========================================
+ Coverage   63.87%   64.83%   +0.96%     
==========================================
  Files         108      108              
  Lines       13865    13865              
==========================================
+ Hits         8856     8990     +134     
+ Misses       4140     3982     -158     
- Partials      869      893      +24     

Flag	Coverage Δ
e2e-test	36.67% <ø> (ø)
e2e-test-oncluster	30.49% <ø> (-0.06%)	⬇️
e2e-test-oncluster-runtime	26.89% <ø> (?)
e2e-test-runtime-python	26.55% <ø> (?)
e2e-test-runtime-quarkus	26.68% <ø> (?)
e2e-test-runtime-rust	25.57% <ø> (?)
e2e-test-runtime-springboot	25.71% <ø> (?)
e2e-test-runtime-typescript	26.68% <ø> (?)
integration-tests	51.79% <ø> (+2.00%)	⬆️
unit-tests-macos-latest	48.75% <ø> (ø)
unit-tests-ubuntu-latest	?
unit-tests-windows-latest	48.78% <ø> (ø)

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[davidhadas]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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