`.github/workflows/deploy_unified_header.yml` needs `permissions: contents: write`

Due to

https://www.stepsecurity.io/blog/hackerbot-claw-github-actions-exploitation

we'll be changing the default GHA permissions for boostorg to read-only.

Since `.github/workflows/deploy_unified_header.yml` does `git push`, it will need to be updated to contain

```
permissions:
contents:
write
```
(https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#permissions)

I'd also recommend enabling branch protection, so that the `master` and `develop` branches can't be rewritten or deleted.