Improve helm chart annotation building script

[jedcunningham]

Very minor improvement, but this gets rid of a bunch of extra newlines and properly indents the annotations, allowing for an append redirect right into Chart.yaml.

Before:

$ python3 ./dev/chart/build_changelog_annotations.py
- description: Don't overwrite ``.Values.airflowPodAnnotations``
  kind: fixed
  links:
  - name: '#37917'
    url: https://github.com/apache/airflow/pull/37917

- description: Fix cluster-wide RBAC naming clash when using multiple ``multiNamespace``
    releases with the same name
  kind: fixed
  links:
  - name: '#37197'
    url: https://github.com/apache/airflow/pull/37197

- description: 'Misc: Default airflow version to 2.8.3'
  kind: changed
  links:
  - name: '#38036'
    url: https://github.com/apache/airflow/pull/38036

After:

$ python3 ./dev/chart/build_changelog_annotations.py                 
    - description: Don't overwrite ``.Values.airflowPodAnnotations``
      kind: fixed
      links:
      - name: '#37917'
        url: https://github.com/apache/airflow/pull/37917
    - description: Fix cluster-wide RBAC naming clash when using multiple ``multiNamespace``
        releases with the same name
      kind: fixed
      links:
      - name: '#37197'
        url: https://github.com/apache/airflow/pull/37197
    - description: 'Misc: Default airflow version to 2.8.3'
      kind: changed
      links:
      - name: '#38036'
        url: https://github.com/apache/airflow/pull/38036