[DBAL-944] Fix table column alteration on DB2

Replacement for #633 as it was messed up, incomplete, untested and wrong.
I decided to go for `protected` to allow inheriting DB2 platforms using it. If we add more platform version variants of DB2 and need to alter `getAlterTableSQL()`, this part can be reused eventually. But if you want to have it `private` I can also do so for now.