Fixes#98769.
Important changes from https://gstreamer.freedesktop.org/releases/1.18/:
* `gst-validate` was renamed to `gst-devtools` upstream:
> * the `gst-validate` tarball has been superseded by
> the `gst-devtools` tarball for consistency with the git module name.
* `gst-python` is now Python 3 only:
> * Python 2.x is no longer supported
