Revert "Allow for both hexcolors with alpha as shorthand"

This reverts commit a1932a5.

Author: vmcj

webapp/src/Utils/Utils.php

@@ -290,10 +290,7 @@ public static function timeStringDiff(string $time1, string $time2): string
      */
     public static function convertToHex(string $color): ?string
     {
-        if (preg_match('/^#[[:xdigit:]]{3,4}$/', $color) ||
-            preg_match('/^#[[:xdigit:]]{6}$/',   $color) ||
-            preg_match('/^#[[:xdigit:]]{8}$/',   $color)
-        ) {
+        if (preg_match('/^#[[:xdigit:]]{3,6}$/', $color)) {
             return $color;
         }