Use yt-dlp instead of youtube-dl

[Th0masL]

This PR is inspired by the issue #237

The symlink idea is taken from tuxpeople

# Output of docker build
> docker build -t th0masl/podsync:ytdlp .
Sending build context to Docker daemon  28.68MB
Step 1/6 : FROM alpine:3.10
 ---> e7b300aee9f9
Step 2/6 : WORKDIR /app
 ---> Using cache
 ---> 6698c8355687
Step 3/6 : RUN wget -O /usr/bin/yt-dlp https://github.com/yt-dlp/yt-dlp/releases/latest/download/yt-dlp &&     chmod +x /usr/bin/yt-dlp &&     ln -s /usr/bin/yt-dlp /usr/bin/youtube-dl &&     apk --no-cache add ca-certificates python3 py3-pip ffmpeg tzdata
 ---> Running in b6877e6396a6
Connecting to github.com (140.82.121.3:443)
Connecting to github.com (140.82.121.3:443)
Connecting to objects.githubusercontent.com (185.199.110.133:443)
yt-dlp               100% |********************************| 2397k  0:00:00 ETA
fetch http://dl-cdn.alpinelinux.org/alpine/v3.10/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.10/community/x86_64/APKINDEX.tar.gz
(1/53) Installing ca-certificates (20191127-r2)
(2/53) Installing sdl2 (2.0.10-r0)
<...>
(53/53) Installing tzdata (2021a-r0)
Executing busybox-1.30.1-r5.trigger
Executing ca-certificates-20191127-r2.trigger
OK: 118 MiB in 67 packages
Removing intermediate container b6877e6396a6
 ---> 260fc9f69e29
Step 4/6 : COPY podsync /app/podsync
 ---> a4c34513713a
Step 5/6 : ENTRYPOINT ["/app/podsync"]
 ---> Running in 65f9823b9b6b
Removing intermediate container 65f9823b9b6b
 ---> 6d2f85326855
Step 6/6 : CMD ["--no-banner"]
 ---> Running in da3bc53600c0
Removing intermediate container da3bc53600c0
 ---> 096ccf41061b
Successfully built 096ccf41061b
Successfully tagged th0masl/podsync:ytdlp

[Th0masL]

Is there anything we want to discuss in order to implement this change ?