Now that ML configurations are stored in the .ml-config
index rather than in cluster state there is a possibility
that some users may try to add configurations directly to
the index. Allowing this creates a variety of problems
including possible data exflitration attacks (depending on
how security is set up), so this commit adds warnings
against allowing writes to the .ml-config index other than
via the ML APIs.
Backport of #38509
David Roberts
lcawl