docs(UPGRADE.md): add note about Inspect API changes

[michaelbeaumont]

Summary

Technically a breaking change... do we think it's worth mentioning?

Changelog: feat(inspect): add gateways to policy inspect

[codecov-commenter]

Codecov Report

Merging #4125 (21695b6) into master (e88a16e) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4125      +/-   ##
==========================================
+ Coverage   55.72%   55.75%   +0.03%     
==========================================
  Files         932      932              
  Lines       56224    56224              
==========================================
+ Hits        31331    31350      +19     
+ Misses      22416    22394      -22     
- Partials     2477     2480       +3     

Impacted Files	Coverage Δ
pkg/defaults/components.go	85.18% <0.00%> (-3.71%)	⬇️
pkg/xds/generator/direct_access_proxy_generator.go	90.80% <0.00%> (+1.14%)	⬆️
pkg/insights/resyncer.go	74.07% <0.00%> (+2.46%)	⬆️
pkg/core/resources/manager/cache.go	85.71% <0.00%> (+2.59%)	⬆️
pkg/kds/reconcile/reconciler.go	84.61% <0.00%> (+5.12%)	⬆️
pkg/events/eventbus.go	92.59% <0.00%> (+7.40%)	⬆️
pkg/kds/client/sink.go	67.56% <0.00%> (+8.10%)	⬆️
pkg/insights/components.go	100.00% <0.00%> (+30.00%)	⬆️

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