[mergebot] Flatten workflows into jobs, fix bugs (#92097)
* flatten the workflows into just jobs in order to give more specific links (link to the specific job that failed instead of just pull), this should make it easier to implement bypass certain failures in the future
* try catch of MandatoryChecksMissingError from find_matching_merge_rule should fix error where merge loops instead of raising runtime error when trunk job fails
* remove usage of on_green and mandatory_only flags just in case. on_green and force are the only two behaviors we currently use
* fail if ghstack pr has non ghstack change, tested locally with #92177 but unsure how to write tests b/c requires use of repo._run_git
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92097
Approved by: https://github.com/huydhn, https://github.com/ZainRizvi
