Update readme post-2.9

blocked by #2 (closed)

changes:

• "it's the same as the Windows version" not true anymore
• Windows 8.1 EOL
• s/.NET Core/.NET 6 Runtime (a.k.a. .NET Core)/
• Lua on Linux
• supported Windows versions
• Linux needs libspeexdsp not anymore, but it does need Lua
• new/graduating cores in table
• add OpenSSF Best Practices badge (see also #47)
• merge "related projects" section into "support" and add link to Reddit; also support.md
• screenshots!

also badges for later