refactor: replace strpos with PHP 8.x string functions

- IRCClient: strpos(..., 'PONG') !== 0 → ! str_starts_with(...)
- IGDBService: strpos(, '//') === 0 → str_starts_with(...)
- Settings: strpos(, '.') !== false → str_contains(...)
- NntmuxCheckIndex: 3x strpos → str_contains

strpos with offset (YencService, MovieService, ReleaseRemoverService)
is left untouched as it is a legitimate use case.

app/Models/Settings.php

@@ -176,7 +176,7 @@ class Settings extends Model
         // Convert numeric strings to actual numbers
         if (is_numeric($value)) {
             // Check if it's an integer or float
-            if (strpos((string) $value, '.') !== false) {
+            if (str_contains((string) $value, '.')) {
                 return (float) $value;
             }