[stable9] Use CLOB for timezone

TEXT defaults to a length of 255 which is going to fail in some cases as the timezone can be rather long.

This changes it back to a CLOB as it has been before as well: owncloudarchive/calendar@8d8bb68. I'm not super convinced that CLOB is the best choice here but at least it seems to work.

Fixes #22876

Backport of https://github.com/owncloud/core/pull/22878 to stable9