osk-layouts: Replace "SS" extra key with "ẞ"

The on-screen keyboard only handles a single keyval per key, so the
current upper-case version of the German "ß" ends up as "S" instead
of the expected "SS".

It is possible to change the keyboard code to emulate multiple key
presses/releases for that particular case, but then luckily a proper
upper-case form exists nowadays: "ẞ".

That seems more appropriate for a single key than a dedicated "SS"
key, so replace it in all layouts that include it. Anybody who prefers
the traditional "SS" can easily tap "S" twice.