Release 6.20.0: Some columns are too short for OAuth access tokens and secrets

[changerequest]

Some access tokens for OAuth accounts are quite long. The first guess how long they can be did not cover that.

Now the UpdateTask
com.openexchange.oauth.internal.groupware.OAuthCre ateTableTask2 extends the columns accessToken and accessSecret to TEXT types. This is a schema blocking update task because the database schema needs to be changed before access is granted to that schema. This task should run quite fast because that table is new to 6.20 release and should not contain that much data.