chore(kubernetes): migrate away from deprecated env variable names (#…

…3355)

Based on the following log warnings:

```
2024-12-03 18:56:06,497 WARN  [org.keycloak.services] (main) KC-SERVICES0110: Environment variable 'KEYCLOAK_ADMIN' is deprecated, use 'KC_BOOTSTRAP_ADMIN_USERNAME' instead
2024-12-03 18:56:06,497 WARN  [org.keycloak.services] (main) KC-SERVICES0110: Environment variable 'KEYCLOAK_ADMIN_PASSWORD' is deprecated, use 'KC_BOOTSTRAP_ADMIN_PASSWORD' instead
```

kubernetes/loculus/templates/keycloak-deployment.yaml

@@ -73,9 +73,9 @@ spec:
                 secretKeyRef:
                   name: keycloak-database
                   key: password
-            - name: KEYCLOAK_ADMIN
+            - name: KC_BOOTSTRAP_ADMIN_USERNAME
               value: "admin"
-            - name: KEYCLOAK_ADMIN_PASSWORD
+            - name: KC_BOOTSTRAP_ADMIN_PASSWORD
               valueFrom:
                 secretKeyRef:
                   name: keycloak-admin