Fix leak of extension-controlplane-shoot-webhooks ManagedResource

[timuthy]

How to categorize this PR?

/area ops-productivity
/area usability
/kind bug

What this PR does / why we need it:
This PR eliminates a race condition that led to a leaked managed resource extension-controlplane-shoot-webhooks during shoot deletion (see #8688 for more details).
Unlike before, the said ManagedResource is only updated when all shoot namespaces are reconciled. Creation won't happen anymore in this scope.

Which issue(s) this PR fixes:
Fixes #8688

Special notes for your reviewer:
/cc @Kostov6 @ialidzhikov

Release note:

An issue was fixed that sometimes led to leaked `extension-controlplane-shoot-webhooks` which blocked the shoot deletion.

[rfranzke]

/assign

[Kostov6]

Thanks for the PR!
Just an initial question from my side: isn't it more clearer to use createOnReconcile instead of createOnUpdate because the variable is being used for creation in the context of the Reconcile function in the builder package?

[timuthy]

@Kostov6 thanks for the feedback!

Just an initial question from my side: isn't it more clearer to use createOnReconcile instead of createOnUpdate

I can rename it 👍

Any additional feedback from your side @rfranzke?

[rfranzke]

/lgtm
/approve

[gardener-prow]

LGTM label has been added.

Git tree hash: 12d0d261a0e1fcbd794947a0dec02c85df4b4383

[gardener-prow]

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This pull-request has been approved by: rfranzke

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