Turn the -i/--ignore-version options of pg_dump and pg_dumpall into no-ops:

the server version check is now always enforced.  Relax the version check to
allow a server that is of pg_dump's own major version but a later minor
version; this is the only case that -i was at all safe to use in.

pg_restore already enforced only a very weak version check, so this is
really just a documentation change for it.

Per discussion.