fix(impersonation): rename DefaultService account to ServiceAccount #27084

[alexymantha]

DefaultServiceAccount no longer makes sense and should be named just ServiceAccount instead. This PR only updates the user-facing references, unfortunately we cannot rename the field in the CRDs without a breaking change.

Linked to 327084
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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 63.35%. Comparing base (62670d6) to head (17539db).

Files with missing lines	Patch %	Lines
cmd/util/project.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #27125      +/-   ##
==========================================
+ Coverage   63.33%   63.35%   +0.01%     
==========================================
  Files         415      415              
  Lines       56545    56545              
==========================================
+ Hits        35815    35824       +9     
+ Misses      17350    17344       -6     
+ Partials     3380     3377       -3     

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