[v1beta2] MachinePool align replica counters

[richardcase]

What would you like to be added (User Story)?

As part of the v1beta2 changes for MachinePool we need to align the replica counter fields in the status to be consistent with other CAPI resources like MachineSet/MachineDeployment.

Detailed Description

This means ensuring that the following fields are updated correctly from the machines:

• Replicas
• ReadyReplicas
• AvailableReplicas
• UpToDateReplicas

Anything else you would like to add?

Part of #12178

Label(s) to be applied

/kind feature
/area machinepool

[richardcase]

/assign
/lifecycle active

[fabriziopandini]

@richardcase FYI we usually do not create issues for each subtask in a tracking issue, but up to you