Make migrations RDBMS agnostic

[zabdurakhimov]

This migration fails on PostgreSql database version 12 stating that

alter column slug in table {{%article}} to string(255) NOT NULL ...Exception: SQLSTATE[42601]: Syntax error: 7 ERROR: syntax error (near: "NOT") LINE 1: ...BLE "article" ALTER COLUMN "slug" TYPE varchar(255) NOT NULL

It runs with mysql database with no error though.
A possible solution might be placing a condition for checking and running the query in accordance with db type

[XzAeRo]

There are several things that are mysql specific. Definitely planning to fix this in the next update.