benchmarks/ddosify: Update to 0.15.0

ChangeLog: https://github.com/ddosify/ddosify/releases/tag/v0.15.0

Introducing engine modes:

engine_mode is optional in the config file.

 * distinct-user mode simulates a new user for every iteration.
 * repeated-user mode can use pre-used user in subsequent iterations.
 * ddosify mode is the default mode of the engine. In this mode, the engine
   runs in its max capacity, and does not show user simulation behavior.

Add breaking changes to UPDATING

PR:		270078
Reported by:	0xdutra@gmail.com (maintainer)