Switch master branch to main

[MattiSG]

I already created a main branch. If this PR gets merged, I suggest to not delete master immediately, but to simply switch the base branch to main, and delete master at a future stage, once the transition has been demonstrated to not have negative side effects.

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If merged:

• Switch default branch to main
• Update branch protection rules to protect main instead of master
• Make master read-only to prevent mistakes
• In the future, erase master branch
• Update Core deploy target following Deploy documentation openfisca-core#1211