Fix forward compatibility between react-admin packages

[fzaninotto]

Closes #5979

[djhi]

I think we have more packages depending on ra-core. Shouldn't we fix them as well?

[fzaninotto]

I don't want to, because then we hit problems when a package hasn't been updated since long (e.g. ra-data-localstorage depends on ra-core@^3.9.0). I think putting a ~ in react-admin dependencies and keeping a ^ in cross-package dependencies is the good balance between security and ease of publication.