Drop support for MySQL 5.6 and older and MariaDB 10.2.6 and older

[morozov]

Q	A
Type	improvement
BC Break	yes

Deprecated in #5072.

We cannot just merge the MySQLPlatform and MySQL57Platform classes as we usually do since the MariaDB platform class extends MySQLPlatform as well. This way would receive all the changes from MySQL57Platform.

TODO:

• Introduce a common base class for all Oracle MySQL and MariaDB databases (Forward compatibility for MySQL-like platforms #5098)
• Deprecate MariaDB MariaDB 10.1 and older (Deprecate support for MariaDB 10.1 and older #5097)
• Deprecate MariaDB MariaDB 10.2.6 and older (Deprecate support for MariaDB 10.2.6 and older #5102)
• Move the code implementing operations with foreign keys and needed for MySQL 5.5 and older from the base class to the MariaDB one.