Add a banner warning users about the upcoming EOL for Win ≤8.1 and macOS ≤10.14

13.5 will be the last Windows version, Mozilla bumped the requirement to Windows 10.

We could add a warning to Windows 7 users somewhere, e.g., in about:tor.

We can check the version of the OS with Services.sysinfo.getProperty("version"). It's 10.0 in my Windows 10 VM, and 6.1 in my Windows 7 VM.

Maybe the first versions of Windows 10 also use 6.1, but if that's the case, they're unsupported too.

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