Doc updates for aws iam auth addition and removal

[snarkychef]

Issue #, if available:
https://github.com/aws/eks-anywhere-internal/issues/2455

Description of changes:
Documentation changes to reflect the newly added capabilities of addition or deletion of aws iam authentication to an existing cluster using upgrade workflows. Relevant code changes in #9897

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.98%. Comparing base (13cb4a5) to head (4ba7a97).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9978      +/-   ##
==========================================
+ Coverage   69.97%   69.98%   +0.01%     
==========================================
  Files         680      680              
  Lines       51124    51148      +24     
==========================================
+ Hits        35774    35798      +24     
  Misses      13551    13551              
  Partials     1799     1799              

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