Use https instead of http

Hello,

While I was browsing VLC website ( https://www.videolan.org/vlc ) i noticed thousands of warnings about mixed content (loading http resources on a https website), so i decided to fix it.

Example warning page in chromium-based browser:

• I didnt update w3.org schemes, even though they work with https.
• I found some 404 websites (both on http and https), so i didnt update broken links (might be a good idea to create linkchecker on a CI/CD).
• I tested ~100 random links (unique domains) to see if anything breaks and so far so good. In 2022 most websites have automatic promotion to SSL version of a website, so it was okay, but still a performance hit.
• Replaced // to https://
• Didnt touch doc/ because of encoding issues (at least in german translation) - im too old to fight with ISO-8859 again ;)

Let me know if you are interested in more contributions from me - frontend dev with performance experience. Im using VLC for so long i thought it would be nice to donate some time for the good cause.