Bumb version to 20.15.0 (#4858)

Author: sreichel

app/Mage.php

@@ -206,7 +206,7 @@ public static function getOpenMageVersionInfo(): array
         if (self::getOpenMageMajorVersion() === 20) {
             return [
                 'major'     => '20',
-                'minor'     => '14',
+                'minor'     => '15',
                 'patch'     => '0',
                 'stability' => '', // beta,alpha,rc
                 'number'    => '', // 1,2,3,0.3.7,x.7.z.92 @see https://semver.org/#spec-item-9