Fix bug which leaves stale machine class name on inplace updated machine

[thiyyakat]

What this PR does / why we need it:

Currently, transferMachinesFromOldToNewMachineSet does not set the new machine class's name on the machine object, causing machine.Spec.Class.Name to contain the old machine class's name.

This causes the old machine class to stick around indefinitely because there are machines still referencing it. Additionally, the stale machine class name in the machine's spec prevents changes in the virtual capacity from being picked up for the updated machine.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Please note that newMachineSet.Spec.MachineClass is always empty, which is why the patch uses the value newMachineSet.Spec.Template.Spec.Class.Name.

Before the fix (Running MCM v0.62.1) :

> k get mcc                                                                                                                                               
NAME                                              AGE
shoot--x--preserve-inplace-test2-z1-74949   30m

> k get mc -oyaml| grep class -i -1                                                
  spec:
    class:
      kind: MachineClass
      name: shoot--x--preserve-inplace-test2-z1-74949

After the inplace update:

> k get mcc                                                                            
NAME                                              AGE
shoot--x--preserve-inplace-test2-z1-74949   34m <-- sticks around
shoot--x--preserve-inplace-test2-z1-a3a40   3m6s

> k get mc -oyaml| grep class -i -1                                              

  spec:
    class:
      kind: MachineClass
      name: shoot--x--preserve-inplace-test2-z1-74949 <-- not updated

After the fix (Running PR branch), after inplace update:

> k get mcc         
                                                             
NAME                                              AGE
shoot--x--preserve-inplace-test2-z1-9d9f2   4m10s

> k get mc -oyaml| grep class -i -1                                                                                                                              
  spec:
    class:
      kind: MachineClass
      name: shoot--x--preserve-inplace-test2-z1-9d9f2

Release note:

Patch `machine.Spec.Class.Name` on machine object with the new machine class's name after it undergoes an inplace update.

[aaronfern]

Is it possible to write a unit test for transferMachinesFromOldToNewMachineSet? To ensure that referenced mcs, labels, and mcc are updated

[elankath]

Quick Q: Is there an unit-test or integration test for this ?

[thiyyakat]

There weren't tests specifically for this. I've added unit tests now on @aaronfern 's suggestion.
Please see commit: a8e519b.

IT for inplace is under review: #1144

[aaronfern]

Thanks for the changes!
/lgtm

[gardener-prow]

LGTM label has been added.

Details

Git tree hash: c7ddf3d72cb9f3e710356c87ebd31f82a4e0801c

[gardener-prow]

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