fix(dependencies): move mockfs to the right package.json (#35476)

Summary:
While working on 0.71 we noticed that we were hitting a "mock-fs not found" issue on CI, caused by the fact that the dependency was set in the wrong spot.

This PR backports to main the fix: https://github.com/facebook/react-native/commit/05646f8f38b65abb9487a348d0ba7e02a35751d9 & https://github.com/facebook/react-native/commit/ceaebc69759e59bfc465722924fee98f2deac825

This is related to the black magics of repo-config, and the fact that devDeps from repo-config don't get propagated back to the root package.json when on stable branch. Because of that, this commit doesn't NOT have a yarn.lock entry (the dep is there in main).

I'm confused as to how that dep ended in the devDeps of repo config, it seems something went wrong in migrating some commits from GH to monorepo and back?
check out:
* https://github.com/facebook/react-native/pull/34580
* https://github.com/facebook/react-native/commit/f0ffd2291c850682e293d9a9ca3dbe7f46bf2e25

## Changelog

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[Internal] [Changed] - move mockfs to the right package.json

Pull Request resolved: https://github.com/facebook/react-native/pull/35476

Test Plan: N/A, change is transparent

Reviewed By: cortinico, cipolleschi, dmytrorykun

Differential Revision: D41530265

Pulled By: jacdebug

fbshipit-source-id: ec7a6426d9a2a766a98a3cccc339ca7ca090c3a0