add preliminary 7.6 upgrade guide
Created by: bbodenmiller
Add preliminary 7.6 upgrade guide. Makes it easier to add upgrades as changes are made rather than trying to round up everything at RC1.
Initial additions:
- Nginx changes needed again in 7.6 as they did not make the final 7.5 upgrade guide
- Suggest that user sets time zone (added in https://github.com/gitlabhq/gitlabhq/pull/8015 but missed in final 7.5 upgrade guide)