Default buffer type to 'heap' for 9.0

[andsel]

Release notes

Switch the default value of pipeline.buffer.type to use the heap memory instead of direct one.

What does this PR do?

Change the default value of the setting pipeline.buffer.type from direct to heap and update consequently the documentation.

Why is it important/What is the impact to the user?

It's part of the work to make more explicit the usage of memory used by Netty based plugins. Using heap instead of direct for Netty buffers, make easier the debug of memory issues at the cost of bigger heap size.

Checklist

• [x] My code follows the style guidelines of this project
• ~~[ ] I have commented my code, particularly in hard-to-understand areas~~
• [x] I have made corresponding changes to the documentation
• [x] I have made corresponding change to the default configuration files (and/or docker env variables)
• ~~[ ] I have added tests that prove my fix is effective or that my feature works~~

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Relates #16353

Use cases

As a developer of Netty based plugin I want to have better insight in memory usage patterns.