ci/build-win32.ps1: bump ffmpeg to 8.1 branch

DOCS/compile-windows.md

@@ -76,7 +76,7 @@ which bootstraps a MinGW-w64 toolchain and builds mpv along with its dependencie
    cat <<EOF > subprojects/ffmpeg.wrap
    [wrap-git]
    url = https://gitlab.freedesktop.org/gstreamer/meson-ports/ffmpeg.git
-   revision = meson-8.0
+   revision = meson-8.1
    depth = 1
    [provide]
    libavcodec = libavcodec_dep

ci/build-win32.ps1

@@ -227,7 +227,7 @@ $projects = @(
     @{
         Path = "$subprojects/ffmpeg.wrap"
         URL = "https://gitlab.freedesktop.org/gstreamer/meson-ports/ffmpeg.git"
-        Revision = "meson-8.0"
+        Revision = "meson-8.1"
         Provides = @(
             "dependency_names = libavcodec, libavdevice, libavfilter, libavformat, libavutil, libswresample, libswscale"
             "program_names = ffmpeg"