[DOCS] Rolling upgrade with old internal indices (#37184)
[DOCS] Rolling upgrade with old internal indices (#37184)

Upgrading the Elastic Stack perfectly documents the process to
upgrade ES from 5 to 6 when internal indices are present. However,
the rolling upgrade docs do not mention anything about internal indices.

This adds a warning in the rolling upgrade procedure, highlighting that
internal indices should be upgraded before the rolling upgrade procedure
can be started.
strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra