fix for mysql automatic column type with high field limit

[i-just]

Description

When using MySQL and creating a Plain Text field, if you set the Field Limit to a high value like 20,000 characters, the following error will be triggered:

SQLSTATE[42000]: Syntax error or access violation: 1074 Column length too big for column 'field_aaa_xxaevbok' (max = 21845); use BLOB or TEXT instead
The SQL being executed was: ALTER TABLE `content` ADD `field_aaa_xxaevbok` varchar(80000)

This PR keeps the current behaviour for PostgreSQL and adjusts PlainText::getContentColumnType() to account for various MySQL limits to use text, medium text and long text columns.

Related issues

support issue