Fix remaining empty annotations: keys in chart templates

Follow-up to #486 which fixed empty annotation blocks
across almost everywhere, but we still have 3 more:

• webhooks/service.yaml: the outer conditional included
  prometheus.webhooks.enabled without the
  not serviceMonitor.enabled guard that manager/service.yaml and
  metrics-server/service.yaml both have. When prometheus + serviceMonitor
  are both enabled (prometheus.io/* annotations aren't needed), the
  annotations block renders empty.

• metrics-server/serviceaccount.yaml: annotations: rendered
  unconditionally, no outer guard at all.

• webhooks/serviceaccount.yaml: same as above.

Empty annotations: keys cause issues with ArgoCD's ServerSideApply —
the apply controller claims ownership of the annotations field, and then
any annotation injected by external controllers (e.g. GKE NEG
controller adding cloud.google.com/neg) is treated as drift, keeping
the app permanently OutOfSync.

How to reproduce

# webhooks service — needs prometheus + serviceMonitor both enabled
helm template keda keda \
  --set prometheus.webhooks.enabled=true \
  --set prometheus.webhooks.serviceMonitor.enabled=true \
  -s templates/webhooks/service.yaml | grep -C1 'annotations:$'

# serviceaccounts — empty annotations with default values
helm template keda keda -s templates/metrics-server/serviceaccount.yaml | grep -C1 'annotations:$'
helm template keda keda -s templates/webhooks/serviceaccount.yaml | grep -C1 'annotations:$'

Checklist

• I have verified that my change is according to the deprecations & breaking changes policy
• Commits are signed with Developer Certificate of Origin (DCO - learn more)
• README is updated with new configuration values (if applicable) learn more
• A PR is opened to update KEDA core (repo) (if applicable, ie. when deployment manifests are modified)

Fixes #397 (although that particular file is already fixed, so should probably be just closed)