Fix 1.1 dumping

[skshetry]

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

Test PR will later work on top of #4295

[skshetry]

Tests passed. Thing to do here are:

1. work this on top of force YAML 1.1 syntax when dumping yaml files with ruamel #4295 (with only dump in 1.1 in dvc.lock files and .dvc files).
2. ask for feedback.
3. See how much this hurts the compatibility.
4. If I don't find anything new and is compatible, release a new minor version (patch won't do).
5. Fix json loading.
6. Document that dvc.yaml is expected to be "1.1 and 1.2 compatible", i.e. compatible subset of either two, and
   ".dvc file" is expected to be in 1.1.

@efiop, it'd be good to get quick feedback on this PR as it is.

Closing for now. See you on the other side, #4295 that is.