Finish replacing youtube-dl with yt-dlp

[mm503]

Hi @mxpv,

I finished replacing YouTube-dl with yt-dlp since YouTube-dl had a last release nearly 4 years ago. I updated the textual references but left the config and internal variables untouched to retain compatibility with configurations out there.

While doing that, I also moved yt-dlp from /usr/bin to /usr/local/bin, which is better suited for external bins (one good use case is that you can easily override the whole /usr/local/bin directory in Kubernetes/Docker and add your own stuff there without affecting system bins).

And finally, I also changed the yt-dlp file permissions to 777 so the self-update feature works even for people who don't run the container as root.

I've been running the test build at home along with my other PR and it works well.

Log fragment:

│ time="2025-03-22T17:53:42Z" level=info msg="skipping due to max_age filter (223d > 14d)" episode_id=vtMy5l0rLx8 filter=max_age                                                                                                                                                  │
│ time="2025-03-22T17:53:42Z" level=info msg="skipping due to max_age filter (167d > 14d)" episode_id=xvDmR2n_D6E filter=max_age                                                                                                                                                  │
│ time="2025-03-22T17:53:42Z" level=info msg="skipping due to already downloaded" episode_id=y85OjRF5Ie8                                                                                                                                                                          │
│ time="2025-03-22T17:53:42Z" level=info msg="download count: 1"                                                                                                                                                                                                                  │
│ time="2025-03-22T17:53:42Z" level=info msg="! downloading episode https://youtube.com/watch?v=lDXxDA5uZvw" episode_id=lDXxDA5uZvw index=0                                                                                                                                       │
│ time="2025-03-22T17:53:53Z" level=info msg="creating file: /data/podsync/m-74gear/lDXxDA5uZvw.mp4" name=m-74gear/lDXxDA5uZvw.mp4                                                                                                                                                │
│ time="2025-03-22T17:53:54Z" level=info msg="successfully downloaded file \"lDXxDA5uZvw\"" episode_id=lDXxDA5uZvw index=0                                                                                                                                                        │
│ time="2025-03-22T17:53:54Z" level=info msg="downloaded 1 episode(s)"                                                                                                                                                                                                            │
│ time="2025-03-22T17:53:54Z" level=info msg="running cleaner" count=40 feed_id=m-74gear                                                                                                                                                                                          │
│ time="2025-03-22T17:53:54Z" level=info msg="creating file: /data/podsync/m-74gear.xml" name=m-74gear.xml

[mxpv]

@mm503 thanks for contribution! From what I understand, yt-dlp is a drop in replacement for youtube-dl, right? Can we just symlink ytdlp? Leaving both options on the table. I don't think we need to rename comments for this to work.