Bug 1810150 - stop scheduling pull-request task graphs when there's already a corresponding push graph

When a PR is opened from a branch in the main repo, we used to create a
task graph for both the push event and the pull-request open or
synchronize event, associated with the same commit, and thus essentially
doing the same work twice.

With this change, we skip the pull-request graph, and rely on the push
graph to run the required checks.  This requires corresponding changes
to the mergify config to consider either of complete-pr or complete-push
as good enough to merge a PR.

This reverts commit de10e3fa.

scriptworker's chain of trust verification expects a decision task's
MOBILE_HEAD_REPOSITORY to match the repository URL in metadata.source.
For pull requests that means we shouldn't set metadata.source to the
base repo.

Co-authored-by: Andrew Halberstadt <ahal@mozilla.com>

Treeherder requires this in its ingestion pipeline, so if the pusher
doesn't have an email then use their name instead.

This reverts commit 2db89572.

This reverts commit c2b26663.

In addition to the requirements bump itself, this means:
- bump decision-mobile docker image
- rename job-defaults to task-defaults, jobs to tasks, job-template to task-template
- stop setting parameters that are now handled by taskgraph upstream

now that Android releases are kicked off through ShipIt