chore(release): Remove support for macOS 10.14

macOS 10.14 is EOL by Apple, and brew no longer supports it. We can no longer
build qTox on 10.14 using our brew-based release process. Instead we will
release from 10.15, which loses compatibility with 10.14 with our current build
process.

INSTALL.md

@@ -566,7 +566,7 @@ See [AppArmor] to enable confinement for increased security.
 
 ## OS X
 
-Supported OS X versions: >=10.8. (NOTE: only 10.13 is tested during CI)
+Supported OS X versions: >=10.15.
 
 Compiling qTox on OS X for development requires 2 tools:
 [Xcode](https://developer.apple.com/xcode/) and [homebrew](https://brew.sh).