[Backport 6.11.x] Add priorityClassName to remaining pods

[sourcegraph-release-bot]

Linear issue FEIE-297: Add priorityClassName to remaining pods

• Customer's Kubernetes cluster policy blocks pods from starting if they do not have a priorityClassName in their config.
• We already had support for priorityClassName, but only for ~5 pods, need to add this for all remaining pods
• Added logic so that priorityClassName could be defined once, under the sourcegraph top level key, and / or under each pod's top-level key, which would override the config on the sourcegraph top level key, so the customer could configure:

sourcegraph:
  priorityClassName: p2

pgsql:
  priorityClassName: p1

Checklist

• Follow the manual testing process
• Update changelog
• Update Kubernetes update doc

Test plan

• Tested with Helm template
• Followed the manual testing process
• Deployed it on my test cluster, with the following override file:

priorityClasses:
- name: test
  value: 100
  preemptionPolicy: Never
  description: "test"
- name: test2
  value: 102
  preemptionPolicy: Never
  description: "test2"

sourcegraph:
  image:
    defaultTag: 6.10.3349
    useGlobalTagAsDefault: true
  priorityClassName: test

• Then re-applied, adding:

gitserver:
  priorityClassName: test2

• Both worked, output:

[2025-12-15 03:58:52] config % kubectl get pods -o custom-columns=NAME:.metadata.name,PRIORITY_CLASS:.spec.priorityClassName,PRIORITY_VALUE:.spec.priority
NAME                                         PRIORITY_CLASS   PRIORITY_VALUE
blobstore-579cbc4cb9-2gn69                   test             100
codeinsights-db-0                            test             100
codeintel-db-0                               test             100
gitserver-0                                  test2            102
gitserver-1                                  test2            102
grafana-0                                    test             100
indexed-search-0                             test             100
pgsql-0                                      test             100
precise-code-intel-worker-5b6bd8d898-9zrbg   test             100
prometheus-65468d765d-j4rgw                  test             100
redis-cache-595c746f84-2wxtf                 test             100
redis-store-5f4b87dbf4-8n24m                 test             100
searcher-0                                   test             100
sourcegraph-frontend-677d647479-77zrl        test             100
syntect-server-657b89b6f4-p59x6              test             100
worker-6d68db5b5c-twxkk                      test             100
``` <br> Backport a49fce293f7dbe87b52fdbd534aa909caecc9a60 from #778

[marcleblanc2]

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