Update 6.x-or-7.x-to-7.6.md
Added missing libkrb5-dev as mentioned here https://github.com/gitlabhq/gitlabhq/edit/7-6-stable/doc/update/7.5-to-7.6.md
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Added missing libkrb5-dev as mentioned here https://github.com/gitlabhq/gitlabhq/edit/7-6-stable/doc/update/7.5-to-7.6.md