Allow merge request updates to cancel running builds from the same source branch (#1901)

* add feature to allow merge request from the same branch to cancel running job that was triggered by the same merge request

* fix(test): Simplify PendingBuildsHandlerTest

* update help link for 'Cancel running merge request builds on update' in config.jelly

* fix: resolve help file loading for cancel running builds option

* refactor: rename PendingBuildsHandler to MergeRequestBuildHandler

The handler cancels both queued and running MR builds, so the old name was misleading. Also remove the redundant nine-parameter factory overload that always defaulted cancelRunningBuildsOnUpdate to false.

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Co-authored-by: hu wang <hu.wang@dematic.com>
Co-authored-by: Manjunath M <A0096308@kiongroup.com>
Co-authored-by: Kris Stern <88480540+krisstern@users.noreply.github.com>