Add webhook trigger when target branch is updated.

The existing webhook supports various triggers, such as when a pull request is opened or when a pull request's source branch has been updated. A trigger for when the pull request's target branch has been updated is missing. 

Recently the 'source branch updated' trigger has been added (see https://jira.atlassian.com/browse/BSERV-10279), but as Martin Herren pointed out here, this solves only half the problem.

A 'target branch updated' trigger is essential for a proper CI/CD workflow, because in order to know whether it is safe to merge a pull request to the target branch, we need to know that the combination of the two branches does not break any functionality or make any tests fail. In a CI/CD workflow, whenever a pull request has been merged to the main branch, this may affect other, pending pull requests that need to be merged to that same target branch. Therefore, a change to the target branch requires a rebuild and re-run of all unit tests of those other, pending pull requests.

The payload for the 'target branch updated' trigger can be identical to that of the 'source branch updated' trigger.