GAE mode is not used in PCT at the moment, and we have exactly zero test automation for it. But it is still a maintenance burden to fix upper bounds dependencies there and to process Dependabot PRs, I suggest we just remove this mode

Old TODO comments from the repo:

1. Export everything to GAE, dropping the data storing in XML files (which pollutes the filesystem and can be easily delete if we are careless) and processing with XSL. (migration already started to GAE datastorage, but not completely finished, especially on build logs). (jglick: this is undesirable, need to be able to review local results without uploading them)
2. Improve GAE app to allow plugin maintainers to subscribe to notifications on plugin compatibility tests for their plugins against new jenkins versions released.
3. Remove possibility, on GAE app, to select both "every plugins" and "every cores" results... because it generates too much results and crash GAE datastore