Relax node version constraint: >=12.13.1
Relax node version constraint: >=12.13.1

The strictness is causing problems for external contributors.

We now have `nvm` installed on the live & staging servers, and we have the version pinned with `.nvmrc`. Internally we should all stick to using the version in `.nvmrc`.

owid/owid-grapher,OurWorldInData/owid-grapher,owid/owid-grapher,owid/owid-grapher,owid/owid-grapher,OurWorldInData/owid-grapher,OurWorldInData/owid-grapher,owid/owid-grapher,OurWorldInData/owid-grapher,OurWorldInData/owid-grapher