Resolve #3500902 "Drupal 11 compatibility"

Closes #3500902

src/Encoder/PlaintextEncoder.php

@@ -27,14 +27,14 @@ class PlaintextEncoder implements EncoderInterface, DecoderInterface {
   /**
    * {@inheritdoc}
    */
-  public function supportsEncoding($format) {
+  public function supportsEncoding($format): bool {
     return $format == static::$format;
   }
 
   /**
    * {@inheritdoc}
    */
-  public function supportsDecoding($format) {
+  public function supportsDecoding($format): bool {
     return $format == static::$format;
   }
 
@@ -43,7 +43,7 @@ class PlaintextEncoder implements EncoderInterface, DecoderInterface {
    *
    * Uses HTML-safe strings, with several characters escaped.
    */
-  public function encode($data, $format, array $context = []) {
+  public function encode($data, $format, array $context = []): string {
     switch (gettype($data)) {
       case "array":
         break;
@@ -125,7 +125,7 @@ class PlaintextEncoder implements EncoderInterface, DecoderInterface {
   /**
    * {@inheritdoc}
    */
-  public function decode($data, $format, array $context = []) {
+  public function decode($data, $format, array $context = []): mixed {
     $results = [];
     return $results;
   }