Doc: MySQL plugin compilation instructions are outdated

https://doc.qt.io/qt-5/sql-driver.html#qmysql currently asks the user to install MySQL 8.0 followed by its C connector.

However, as of MySQL 8.0.19.0, the C API is not separately installable anymore; it comes bundled with the server [1]. The default folders to use are:

• MYSQL_INCDIR="C:/Program Files/MySQL/MySQL Server 8.0/include"
• MYSQL_LIBDIR="C:/Program Files/MySQL/MySQL Server 8.0/lib"

Furthermore,

• MySQL 8.0 doesn't provide 32-bit C libraries
• The Qt docs refer to MySQL 6.1 which doesn't exist [2] (Ignore that; the C connector has a different version numbering system [3])

(Please double-check this information before updating the docs. It's my first time building the MySQL plugin for Qt)

[1] https://dev.mysql.com/doc/refman/8.0/en/c-api-implementations.html
[2] https://en.wikipedia.org/wiki/MySQL#Release_history
[3] https://downloads.mysql.com/archives/c-c/