Add Breaking changes documentation related to setting pipeline.buffer.type to heap by default

[robbavey]

#16353 introduced changes for 9.0 to change the default value of pipeline.buffer.type to heap instead of the previous value of direct.

This may have require users to change heap settings and direct memory allocations in their jvm.options file, specifically users using the beats, http and/or tcp inputs who are currently using a significant amount of heap, or have customized their JVM allocations to increase the amount of direct memory in use