bug: disallow updating apiServerLB private IP after AzureCluster creation

[3mbe]

What type of PR is this?
/kind bug

What this PR does / why we need it:

• Forbids updating the API server LB private IP after AzureCluster creation.
• Applies to type: Internal and type: Public when feature.APIServerILB is enabled.
• Prevents control-plane endpoint disruption.

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 #5314

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• cherry-pick candidate

Release note:

Forbid changing the API server LoadBalancer private IP after AzureCluster creation (including Public+APIServerILB); updates now return FieldValueForbidden.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @3mbe. Thanks for your PR.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.97%. Comparing base (3decd56) to head (8151715).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5827      +/-   ##
==========================================
+ Coverage   46.94%   46.97%   +0.02%     
==========================================
  Files         279      279              
  Lines       29687    29697      +10     
==========================================
+ Hits        13936    13949      +13     
+ Misses      14938    14936       -2     
+ Partials      813      812       -1     

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[willie-yao]

/assign @jackfrancis

[bryan-cox]

This code logic is duplicated in two places. Maybe you can make a helper function to reduce the duplication and improve the maintainability.

[bryan-cox]

Question: Should this immutability check apply regardless of the feature gate state? Once a private IP is set, it probably shouldn't be changed even if the feature gate changes.