Filter reactor FieldErrors on validation to only error-level alerts.

[evankanderson]

See knative/serving#13399 (comment)

When we added warning-level api.FieldErrors, we missed filtering the warnings from ValidateCreates and ValidateUpdates.
For knative/serving#13399, we want to be able to issue warnings on insecure default values for containers, but we don't want to force all our existing tests to use secure default values (and we want to validate that the insecure values will still work for controllers, etc).

[codecov]

Codecov Report

Base: 81.33% // Head: 81.28% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (1b1ea86) compared to base (714b763).
Patch coverage: 61.90% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2611      +/-   ##
==========================================
- Coverage   81.33%   81.28%   -0.05%     
==========================================
  Files         162      162              
  Lines        9824     9845      +21     
==========================================
+ Hits         7990     8003      +13     
- Misses       1595     1603       +8     
  Partials      239      239              

Impacted Files	Coverage Δ
metrics/config_observability.go	80.35% <61.90%> (-11.08%)	⬇️

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[evankanderson]

/assign @psschwei @mattmoor

[mattmoor]

/lgtm
/approve

[knative-prow]

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