[TASK] Add missing btn-default class for confirm cancel button

Cancel button of the confirm modal missed the btn-default class.

Resolves: #70277
Releases: master
Change-Id: I6b23f828524f824184b09948c3ed3216ec4dba7d
Reviewed-on: http://review.typo3.org/44045


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>