doc: migration: Move picolibc section to required changes

After having migrated a bunch of apps to the new default C library we
have realized that some of the caveats described in the existing
picolibc section of the migration guide do require action on the user
more often than previously thought. In order to ensure that all users
see those and can act upon them, move the picolibc migration info to the
required changes section.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>