Add workers.celery.serviceAccount & workers.kubernetes.serviceAccount

[Miretpl]

related: #28880

This PR introduces two new sections: workers.celery.serviceAccount and workers.kubernetes.serviceAccount. The workers.serviceAccount section is now deprecated.

In terms of logic, it works a little differently than other fields/sections moved to the workers.celery/workers.kubernetes section. All fields under workers.celery override values under workers section, but when workers.kubernetes section is used, the Service Account is generated from templates/workers/worker-kubernetes-serviceaccount.yaml file instead of templates/workers/worker-serviceaccount.yaml. Despite that separation, some fields have fallbacks to the workers section to match behaviour of other, already moved, fields/sections.

Additional change - all ServiceAccount was changed to Service Account in values.yaml as that was missed.

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.