Replace references to the main branch in release branches

[NickLarsenNZ]

Currently the stacks and demos refer to files in the main branch.
In the current state, demos/stacks are not reproducable for a release (apart from the latest stable).

This causes the following problems:

• PRs to main must wait until release time to be merged (otherwise demos could break for the stable release)
• Testing release upgrades becomes tricky, because it deploys main manifests which might have changed and now appear to be broken for the previous release.

This should be automatable, and maybe done via a script in stackable-utils/release

### Releases fixed
- [ ] https://github.com/stackabletech/demos/pull/74
- [ ] https://github.com/stackabletech/demos/pull/73
- [ ] https://github.com/stackabletech/demos/pull/72
- [x] 23.7 (there are no stacks/demos)
- [x] 23.4 (there are no stacks/demos)
- [x] 23.1 (there are no stacks/demos)