Change immutable field errors to type Forbidden

[mitalipaygude]

Issue #, if available:

Description of changes: Change immutable field errors to type Forbidden instead of Invalid so as to follow CAPI.

Testing (if applicable):

• Unit tests

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[codecov[bot]]

Codecov Report

Merging #2904 (633cb03) into main (bad9ce2) will increase coverage by 0.00%. The diff coverage is 94.44%.

@@           Coverage Diff           @@
##             main    #2904   +/-   ##
=======================================
  Coverage   62.48%   62.48%           
=======================================
  Files         335      335           
  Lines       26991    26995    +4     
=======================================
+ Hits        16865    16869    +4     
  Misses       8839     8839           
  Partials     1287     1287           

Impacted Files	Coverage Δ
pkg/api/v1alpha1/cluster_webhook.go	86.33% <84.61%> (+0.09%)	:arrow_up:
pkg/api/v1alpha1/awsiamconfig_webhook.go	73.80% <100.00%> (ø)
...api/v1alpha1/cloudstackdatacenterconfig_webhook.go	79.51% <100.00%> (+0.24%)	:arrow_up:
pkg/api/v1alpha1/fluxconfig_webhook.go	67.39% <100.00%> (ø)
pkg/api/v1alpha1/gitopsconfig_webhook.go	50.00% <100.00%> (ø)
pkg/api/v1alpha1/oidcconfig_webhook.go	70.83% <100.00%> (ø)
...kg/api/v1alpha1/vspheredatacenterconfig_webhook.go	75.64% <100.00%> (+0.31%)	:arrow_up:
pkg/api/v1alpha1/vspheremachineconfig_webhook.go	88.67% <100.00%> (+0.10%)	:arrow_up:

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[mitalipaygude]

/approve

[eks-distro-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mitalipaygude

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