Fix PHP 8.1 incompatible return type deprecation

The proper fix would require to actually declare the `mixed` return
type, but that is only available as of PHP 7.2.0, so we use the
`ReturnTypeWillChange` attribute, which is ignored prior to PHP 8.1.0.

cmsimple/classes/PluginConfig.php

@@ -48,6 +48,7 @@ public function __construct($language = false)
      *
      * @return bool
      */
+    #[\ReturnTypeWillChange]
     public function offsetExists($offset)
     {
         if (!isset($this->configs[$offset])) {
@@ -63,6 +64,7 @@ public function offsetExists($offset)
      *
      * @return array
      */
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         if (!isset($this->configs[$offset])) {
@@ -79,6 +81,7 @@ public function offsetGet($offset)
      *
      * @return void
      */
+    #[\ReturnTypeWillChange]
     public function offsetSet($offset, $value)
     {
         if (!isset($this->configs[$offset])) {
@@ -94,6 +97,7 @@ public function offsetSet($offset, $value)
      *
      * @return void
      */
+    #[ReturnTypeWillChange]
     public function offsetUnset($offset)
     {
         if (!isset($this->configs[$offset])) {