Fix post-migration warning about index corruption being too scary

[Gargron]

Old (printed into Rails log, which may sometimes be disabled, write to a file, or by default to stdout):

WARNING: Your database is using an unsafe collation setting, which might result in index corruption.
WARNING: See https://docs.joinmastodon.org/admin/troubleshooting/index-corruption/#am-i-affected

New (printed to stderr):

Your database collation is susceptible to index corruption.
  (This warning does not indicate that index corruption has occured and can be ignored)
  (To learn more, visit: https://docs.joinmastodon.org/admin/troubleshooting/index-corruption/)