WL#9722: Deprecate libmysqld embedded server in 5.7

Affects: Server-5.7   —   Status: Complete   —   Priority: Medium

This WL covers deprecation of libmysqld from MySQL 5.7.

User Documentation
==================

* https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-19.html
* https://dev.mysql.com/doc/refman/5.7/en/libmysqld.html
* https://dev.mysql.com/doc/refman/5.7/en/mysql-config.html
* https://dev.mysql.com/doc/refman/5.7/en/mysql-nutshell.html#mysql-nutshell-deprecations
  (includes migration instructions)
No code change, this task is to update the documentation only.

The embedded server is deprecated in 5.7, and removed in 8.0