Issue #3163536 by harishh, ankithashetty, volkswagenchick: License "GPLv2+" is not a valid SPDX license identifier

Also remove xmlrpc as a test dependency. It's not actually used in any
tests, at least not any run by the drupal.org testbot.

Fixing without an issue since it's such a small bug.

Issue #3044467 by wizonesolutions, khaldoon_masud, jasonschweb, Pancho: Webform Signature Field doesn't work with FillPDF

Issue #3060086 by Pancho, wizonesolutions: Introduce new PdfBackend plugins to replace legacy FillPdfBackend and BackendService plugins

Issue #2948793 by wizonesolutions, Liam Morland, liquidcms, noahzenzen: Support PDF security in pdftk

This fixes error reporting when something goes wrong with the underlying
backend.

Issue #3046530 by Pancho, wizonesolutions: Revert: Add experimental sub-module to switch between BC and Next API

Issue #3040900 by Pancho: Move legacy backend support incl. the JavaBridge ("local") backend to a submodule

Issue #3060086 by Pancho, wizonesolutions: Introduce new PdfBackend plugins to replace legacy FillPdfBackend and BackendService plugins

Partially revert: Issue #3059238 by Pancho: Use self:: instead of static:: to avoid late static bindings overshooting

Followup to: Issue #3059238 by Pancho: Inject link manipulator service to avoid late static bindings overshooting