gstreamer: 1.16.2 -> 1.18.0

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 * the `gst-validate` tarball has been superseded by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

pkgs/development/libraries/gstreamer/libav/default.nix

@@ -15,11 +15,11 @@
 
 stdenv.mkDerivation rec {
   pname = "gst-libav";
-  version = "1.16.2";
+  version = "1.18.0";
 
   src = fetchurl {
     url = "${meta.homepage}/src/${pname}/${pname}-${version}.tar.xz";
-    sha256 = "1wpfilc98bad9nsv3y1qapxp35dvn2mvwvrmqwrsj58cf09gc967";
+    sha256 = "0sm0sfdlalimpkf7a7rk7whvyvmmfi2kly2z3q2j5z53x5f3zya2";
   };
 
   outputs = [ "out" "dev" ];
@@ -37,6 +37,15 @@ stdenv.mkDerivation rec {
     libav
   ];
 
+  mesonFlags = [
+    "-Ddoc=disabled" # `hotdoc` not packaged in nixpkgs as of writing
+  ];
+
+  postPatch = ''
+    patchShebangs \
+      scripts/extract-release-date-from-doap-file.py
+  '';
+
   meta = with lib; {
     description = "FFmpeg/libav plugin for GStreamer";
     homepage = "https://gstreamer.freedesktop.org";