fix Cluster -> AzureManagedMachinePool mapper

[nojnhuh]

What type of PR is this?
/kind bug

What this PR does / why we need it:

This PR fixes the mapper used to set up the AzureManagedMachinePoolController to correctly map Cluster events to a particular AzureManagedMachinePool. e.g. Unpausing a Cluster was not triggering a reconciliation for AzureManagedMachinePools associated with that Cluster.

I think what was there before was a plain copy-paste from the AzureManagedControlPlane controller. It has been updated to more closely match how the AzureMachinePool controller is set up.

Related: #3699

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Fixed a bug causing Cluster events not to re-reconcile AzureManagedMachinePools

[nojnhuh]

/cherry-pick release-1.9
/cherry-pick release-1.10

[k8s-infra-cherrypick-robot]

@nojnhuh: once the present PR merges, I will cherry-pick it on top of release-1.9 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.9
/cherry-pick release-1.10

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[codecov]

Codecov Report

Patch coverage: 40.00% and project coverage change: -0.01% ⚠️

Comparison is base (8893daa) 54.77% compared to head (53204c9) 54.76%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3809      +/-   ##
==========================================
- Coverage   54.77%   54.76%   -0.01%     
==========================================
  Files         187      187              
  Lines       19098    19102       +4     
==========================================
+ Hits        10460    10461       +1     
- Misses       8070     8072       +2     
- Partials      568      569       +1     

Files Changed	Coverage Δ
controllers/azuremanagedmachinepool_controller.go	52.91% <40.00%> (-0.56%)	⬇️

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[Jont828]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 1e9f2c896c19b2666ec527e4bcb542915e7d9fb9

[mboersma]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mboersma

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• OWNERS [mboersma]

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[k8s-infra-cherrypick-robot]

@nojnhuh: new pull request created: #3815

In response to this:

/cherry-pick release-1.9
/cherry-pick release-1.10

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[k8s-infra-cherrypick-robot]

@nojnhuh: new pull request created: #3816

In response to this:

/cherry-pick release-1.9
/cherry-pick release-1.10

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.