fix(core): validate both old and new objects on Update (backport #4589)

[michaelbeaumont]

Summary

Backport #4589

[codecov-commenter]

Codecov Report

Merging #4593 (4d8f114) into release-1.6 (d8cdf27) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-1.6    #4593      +/-   ##
===============================================
- Coverage        55.78%   55.75%   -0.03%     
===============================================
  Files              932      932              
  Lines            56249    56250       +1     
===============================================
- Hits             31376    31364      -12     
- Misses           22393    22403      +10     
- Partials          2480     2483       +3     

Impacted Files	Coverage Δ
pkg/api-server/resource_endpoints.go	79.88% <100.00%> (+0.11%)	⬆️
pkg/core/resources/access/admin_resource_access.go	100.00% <100.00%> (ø)
pkg/core/runtime/component/component.go	81.13% <0.00%> (-7.55%)	⬇️
pkg/core/tokens/default_signing_key.go	66.66% <0.00%> (-5.56%)	⬇️
pkg/kds/reconcile/reconciler.go	79.48% <0.00%> (-5.13%)	⬇️
pkg/plugins/runtime/gateway/route/sorter.go	66.66% <0.00%> (-5.13%)	⬇️
pkg/api-server/server.go	85.35% <0.00%> (-2.93%)	⬇️
pkg/plugins/leader/postgres/leader_elector.go	97.87% <0.00%> (-2.13%)	⬇️
pkg/xds/generator/direct_access_proxy_generator.go	89.65% <0.00%> (-1.15%)	⬇️
pkg/core/resources/manager/cache.go	88.31% <0.00%> (+2.59%)	⬆️
... and 2 more

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