fix: explicitly set serviceAccountName in deployment podspec

[ericwyles]

What this PR does / why we need it

This uses the existing archivista.serviceAccountName from helpers to explicitly set the serviceAccountName for pods. Without this change the pods still use the default service account even if a different service account is created by the helm chart when serviceAccount.create is true.

Which issue(s) this PR fixes (optional)

Fixes #370

Acceptance Criteria Met

• Docs changes if needed
• Testing changes if needed
• All workflow checks passing (automatically enforced)
• All review conversations resolved (automatically enforced)
• DCO Sign-off

Special notes for your reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 1.64%. Comparing base (a035c62) to head (0867c9a).
Report is 160 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #371       +/-   ##
==========================================
- Coverage   82.40%   1.64%   -80.76%     
==========================================
  Files          10     120      +110     
  Lines         358   28845    +28487     
==========================================
+ Hits          295     474      +179     
- Misses         43   28314    +28271     
- Partials       20      57       +37     

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[mikhailswift]

Thanks for this fix!