upgrade `symbols/eu` to Version 1.3 (partial)

The author's version 1.3 comes with an important update: the new official Ssharp letter in German language.

Following the author's changelog: https://eurkey.steffen.bruentjen.eu/changelog.html  DONE: Added ✓ and ✗ (replacing © and №) DONE: Added capital ß (ẞ) TODO: Made ¬ a dead key with these bindings ...