Change workflow by changing from 'master' and default 'dev' branches to a single 'main' branch
- [x] In `.gitlab-ci.yml` switch from `master` and `dev` branches to `$CI_DEFAULT_BRANCH` branch
- [x] Update documentation about the new workflow
- Switch references to `master` and `dev` branches into URLs to `main`
- [x] releases notes on the forum
- [x] Push `main` branch forked from `dev` branch
- [x] protect it
- [x] set it as default
- [x] Switch badges to `main`
- [x] Switch target branch of open MRs to `main` branch
- [x] Switch pipeline schedule
- [ ] Unprotect and delete `dev` and `master` branches to avoid any confusion. (done for `master`).
- Unfortunately, we need to keep `dev` branch, since there is links to it from PyPI v0.10.0 release
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