In DOCS-14914, we removed references to the Multiple Key String format in the tutorial for rotating key files here, as the Server Team confirmed that Multiple Key Strings has never been supported format to begin with.

A customer has updated DOCS-14914 and pointed out a remaining reference to Multiple Key Strings: https://www.mongodb.com/docs/manual/core/security-internal-authentication/#keyfile-format

We should also remove this reference as we don't support this format.