Change workflow by changing from 'master' and default 'dev' branches to a single 'main' branch

• In .gitlab-ci.yml switch from master and dev branches to $CI_DEFAULT_BRANCH branch
• Update documentation about the new workflow
  • Switch references to master and dev branches into URLs to main
  • releases notes on the forum
• Push main branch forked from dev branch
  • protect it
  • set it as default
  • Switch badges to main
  • Switch target branch of open MRs to main branch
  • 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