Change preset-env corejs version to '3.5'

[skoging]

Closes #8875

Changed to babel preset env corejs option to '3.5' to match the version that react-app-polyfill depends on, as recommended by core-js.

[Mrman]

Closes #8779 too

[Mrman]

Hi @ianschmitz, if this conflict were to be resolved what's the chance of getting this merged and released with 4.0.0?

[ianschmitz]

The tricky thing about this is there are many projects beyond ours that use this preset. We don't know which minor of core-js they have installed. We could resolve the package and inspect its version, but this wouldn't be foolproof as there could by various versions of the package within the dependency graph.

[Mrman]

Makes sense. Could the breaking change be mitigated if this were a major version release of babel-preset-react-app?

[ianschmitz]

We could potentially do that. We would likely want to also specify a minimum version of core-js in peerDependencies. We should have likely already been doing that. Probably just an oversight.

[Mrman]

I've opened a PR with the peer dependency included but have some observations and questions. They're on the PR: #9021