⏲ `make-release-branch.sh` script needs to replace `HEAD` and `main` with the release branch

[NickLarsenNZ]

Note

This came out of the 25.3.0 Release Retro.

Without this, we get Antora errors about duplicate pages:

node_modules/.bin/antora generate local-antora-playbook.yml
[14:12:28.017] FATAL (antora): Duplicate page: nightly@home:demos:airflow-scheduled-job.adoc
    1: docs/modules/demos/pages/airflow-scheduled-job.adoc in https://github.com/stackabletech/demos.git (branch: main | start path: docs)
    2: docs/modules/demos/pages/airflow-scheduled-job.adoc in https://github.com/stackabletech/demos.git (branch: release-25.3 | start path: docs)

See manual commit made here after troubleshooting the error: #725