fix: make sure machine pools are deleted before gke cluster

[salasberryfin]

What type of PR is this?

/kind bug

What this PR does / why we need it:

During cluster deletion, a GCPManagedCluster may be deleted before its associated GCPManagedMachinePool/s which actually needs the cluster resource to exist to delete successfully:

E0321 10:41:55.730846       1 controller.go:324] "Reconciler error" err="GCPManagedCluster.infrastructure.cluster.x-k8s.io \"cluster-gke\" not found" controller="gcpmanagedmachinepool" controllerGroup="infrastructure.cluster.x-k8s.io" controllerKind="GCPManagedMachinePool" GCPManagedMachinePool="default/cluster-gke-mp-0" namespace="default" name="cluster-gke-mp-0" reconcileID="5e38a1b8-a006-4c22-973b-a7ebe1ca8c06"
E0321 10:42:05.971755       1 gcpmanagedmachinepool_controller.go:270] "Failed to retrieve GCPManagedCluster from the API Server" err="GCPManagedCluster.infrastructure.cluster.x-k8s.io \"cluster-gke\" not found" controller="gcpmanagedmachinepool" controllerGroup="infrastructure.cluster.x-k8s.io" controllerKind="GCPManagedMachinePool" GCPManagedMachinePool="default/cluster-gke-mp-0" namespace="default" name="cluster-gke-mp-0" reconcileID="becd4efd-9abf-4dda-a3fb-ec1e1cfd6d95"

This happens intermittently and it's hard to reproduce, but there needs to be logic that waits for the machine pools to be removed before continuing with deleting the cluster.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

This is similar to this CAPA issue.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Wait for machine pools to be deleted before deleting gke cluster

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[alexander-demicev]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: alexander-demicev, salasberryfin
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[furkatgofurov7]

/lgtm