Set `pipeline.buffer.type` to `heap` by default

[robbavey]

In #16054, we introduced a new setting to allow users to set the allocation of memory for certain input plugins to use either direct or heap memory.

We should consider making this the default setting for Logstash 9.0

### Tasks
- [x] on `8.x`, when `pipeline.buffer.type` is not set, we should emit a deprecation log guiding to do one of two things before upgrading to 9.x: _either_ (1) set `pipeline.buffer.type: heap` and tune heap size upward in advance of upgrading to 9.x _or_ (2) set `pipeline.buffer.type: direct` to lock in the existing behaviour. #16498
- [ ] on `main`, set default for `pipeline.buffer.type` to `heap` #16500