Bump FFmpeg to 8.1.2 in the manylinux2014 i686 build

The other six Dockerfiles on this branch move 8.1.1 -> 8.1.2, but the i686
one was still pinned at 5.1.4. The 4.x counterpart (#1249) covered this file,
so bump it here too to keep the branches consistent.

Author: MDSALMANSHAMS

docker/manylinux2014/Dockerfile_i686

@@ -2,7 +2,7 @@ FROM quay.io/pypa/manylinux2014_i686:latest
 
 ARG CCACHE_VERSION=3.7.9
 ARG CMAKE_VERSION=3.17.0
-ARG FFMPEG_VERSION=5.1.4
+ARG FFMPEG_VERSION=8.1.2
 ARG NASM_VERSION=2.15.04
 ARG OPENSSL_VERSION=1_1_1w
 ARG QT_VERSION=5.15.0