Update isConcurrent RTR option usage (#28546)

Reverting some of https://github.com/facebook/react/pull/27804 which
renamed this option to stable. This PR just replaces internal usage to
make upcoming PRs cleaner.

Keeping isConcurrent unstable for the next major release in order to
enable a broader deprecation of RTR and be consistent with concurrent
rendering everywhere for next major.
(https://github.com/facebook/react/pull/28498)
- Next major will use concurrent root
- The old behavior (legacy root by default, concurrent root with
unstable option) will be preserved for React Native until new
architecture is fully shipped.
- Flag and legacy root usage can be removed after RN dependency is
unblocked without an additional breaking change