gir-files: Import nullable fixes from 1.18 branches

This MR includes preliminary gir-files changes from gstreamer, gstreamer-base and gstreamer-bad from the 1.18 branch, all scheduled for 1.18.2.

trust_return_value_nullability has been introduced for gstreamer, gstreamer-base, gstreamer-net and gstreamer-check and all nullable = false overrides are removed.

This has been tried and reverted for gstreamer-player whose annotations are not ready yet. When that is fixed don't forget to subsequently remove all unnecessary nullable = false overrides in Gir.toml as well!

See this discussion that sparked the creation of this MR.