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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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