gstreamer1.0: upgrade 1.18.5 -> 1.20.0

gstreamer1.0: upgrade 1.18.5 -> 1.20.0

- License update:
  The code actually is LGPLv2.1 but the COPYING file is wrong.
  https://gitlab.freedesktop.org/gstreamer/gstreamer/-/commit/d157b23fb0ce175b167611a4cb019599d9017042

- Drop backported patches
  0001-gst-gstpluginloader.c-when-env-var-is-set-do-not-fal.patch
  0002-Remove-unused-valgrind-detection.patch
  0003-tests-seek-Don-t-use-too-strict-timeout-for-validati.patch

- Update submitted patches
  0001-tests-respect-the-idententaion-used-in-meson.patch
  0002-tests-add-support-for-install-the-tests.patch
  0003-tests-use-a-dictionaries-for-environment.patch
  0004-tests-add-helper-script-to-run-the-installed_tests.patch

Signed-off-by: Jose Quaresma <quaresma.jose@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>