Remove Pulsar Manager support from the chart

[lhotari]

Motivation

Apache Pulsar Manager has been poorly maintained for a long
time — the chart has shipped a warning recommending the Dekaf UI instead for several releases, and the
bundled image is still apachepulsar/pulsar-manager:v0.4.0. Keeping the component in the chart means
maintaining ~600 lines of templates, a dedicated CI cluster job and an extra superuser role for software
that is effectively unmaintained.

This PR drops Pulsar Manager support completely. Dekaf
(components.dekaf: true) is the supported web UI in this chart and the recommended alternative.

Modifications

Chart

• Deleted pulsar-manager-statefulset.yaml, pulsar-manager-service.yaml, pulsar-manager-configmap.yaml,
  pulsar-manager-ingress.yaml, pulsar-manager-admin-secret.yaml and
  pulsar-manager-cluster-initialize.yaml.

• Added pulsar-manager-removed-validation.yaml, which fails the render when components.pulsar_manager
  is truthy so that the removal is explicit rather than silently dropping deployed resources. It follows
  the existing broker-package-storage-validation.yaml pattern (a rendered template, not a _-prefixed
  partial, so the fail is actually executed) and names the keys to remove plus the Dekaf alternative:

  Error: execution error at (pulsar/templates/pulsar-manager-removed-validation.yaml:27:4): ERROR: Pulsar
  Manager support has been removed from the Apache Pulsar Helm chart. Remove the `components.pulsar_manager`
  key from your values (or set it to false), together with the `pulsar_manager` section,
  `images.pulsar_manager` and `auth.superUsers.manager`, to continue. Dekaf is the supported alternative UI
  in this chart: set `components.dekaf: true`. See the 'Pulsar Manager support has been removed' section in
  README.md.
  

• values.yaml: removed components.pulsar_manager, images.pulsar_manager, auth.superUsers.manager
  and the whole pulsar_manager: section.

Docs

• README.md: removed the Pulsar Manager feature bullet and section; the Dekaf UI section now states it is
  the supported web UI and the migration target; new ## Upgrading to Helm chart version 4.8.0 →
  ### Pulsar Manager support has been removed section explains the fail-fast error, that the
  components.pulsar_manager key must be removed or set to false to allow the upgrade, which other stale
  keys to drop, the PVC backup caveat, and how to enable Dekaf.
• examples/README.md and the example values files no longer mention the component.

CI

• Removed the Pulsar Manager cluster matrix job and .ci/clusters/values-pulsar-manager.yaml,
  ci::test_pulsar_manager from .ci/helm.sh and its call site in .ci/chart_test.sh, the
  --set components.pulsar_manager=true kubeconform flag, and the manager blocks in
  .ci/templates-all-values*.yaml, .ci/kustomization.yaml, .ci/values-common.yaml and the cluster
  values files.

Note: Chart.yaml is still at the released 4.7.0 since this repo bumps the version in the release
commit. The README section is written for the next minor, 4.8.0 — it needs adjusting if the next release
number differs.

Verifying this change

• helm lint charts/pulsar passes.
• helm template renders cleanly for the default values, all .ci/clusters/*.yaml (with
  .ci/values-common.yaml), all examples/*.yaml, and .ci/templates-all-values.yaml with and without
  templates-all-values-patch1.yaml.
• --set components.pulsar_manager=true fails with the message above.
• A values file still carrying stale pulsar_manager: / images.pulsar_manager /
  auth.superUsers.manager keys but components.pulsar_manager: false renders fine, so upgrading only
  requires removing or disabling the component flag.
• Rendered superUserRoles no longer contains the manager role.
• Make sure that the change passes the CI checks.