Chart: Support Helm template expressions in podAnnotations (#63019)

* Support Helm template expressions in podAnnotations

Enable tpl evaluation for `podAnnotations` and `airflowPodAnnotations`
across all chart templates, allowing users to use Helm template
expressions such as `{{ .Release.Name }}` or
`{{ include ... | sha256sum }}` in annotation values.

This is useful for adding checksum annotations that trigger pod restarts
when dependent ConfigMaps or Secrets change.

closes: #62698

* Rename chart newsfragment to PR number 63019

* Fix doc nits in customizing-labels: trim underlines and clarify api-server/webserver wording

* Address doc structure: add Customizing Pod Labels subsection under umbrella heading