fix(kuma-cp): memory store cannot delete a parent (backport #4194)

[mergify]

This is an automatic backport of pull request #4194 done by Mergify.

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[codecov-commenter]

Codecov Report

Merging #4196 (1a46939) into release-1.6 (653f198) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-1.6    #4196      +/-   ##
===============================================
+ Coverage        55.73%   55.77%   +0.03%     
===============================================
  Files              932      932              
  Lines            56230    56249      +19     
===============================================
+ Hits             31342    31371      +29     
+ Misses           22404    22399       -5     
+ Partials          2484     2479       -5     

Impacted Files	Coverage Δ
pkg/plugins/resources/memory/store.go	78.80% <100.00%> (+1.08%)	⬆️
pkg/test/store/owner_test_template.go	100.00% <100.00%> (ø)
pkg/xds/cache/once/cache.go	87.17% <0.00%> (-7.70%)	⬇️
pkg/defaults/components.go	88.88% <0.00%> (+3.70%)	⬆️
pkg/kds/client/sink.go	59.45% <0.00%> (+5.40%)	⬆️
pkg/core/tokens/default_signing_key.go	72.22% <0.00%> (+5.55%)	⬆️
pkg/plugins/leader/postgres/leader_elector.go	100.00% <0.00%> (+6.38%)	⬆️
pkg/test/grpc/clientstream.go	92.00% <0.00%> (+8.00%)	⬆️

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