Update dependency net.kyori:adventure-text-serializer-plain to v4.21.0

Author: renovate[bot]

pom.xml

@@ -73,7 +73,7 @@
         <dependency>
             <groupId>net.kyori</groupId>
             <artifactId>adventure-text-serializer-plain</artifactId>
-            <version>4.18.0</version>
+            <version>4.21.0</version>
         </dependency>
         <dependency>
             <groupId>io.papermc.paper</groupId>