updated to Paper 26.2, Java 25 and Gradle 9.6.1

- paper-api 26.2.build.129-stable (all modules), Java toolchain 25, shadow 9.6.1
- fixed NoSuchFieldError on join/quit/chat: Adventure 5 ClickEvent API
  (ClickEvent.suggestCommand / openUrl instead of ClickEvent.Action constants)
- fixed NBTComponent generics in AntiFormattedBook
- replaced javax.annotation with org.jetbrains.annotations (no longer provided by paper-api)
- local.gradle.example: Gradle 9 compatible (archiveFile, providers.exec)

The previous state is tagged as paper-1.21.10.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

common/build.gradle

@@ -1,7 +1,7 @@
 dependencies {
     implementation project(':core')
 
-    compileOnly 'io.papermc.paper:paper-api:1.21.10-R0.1-SNAPSHOT'
+    compileOnly 'io.papermc.paper:paper-api:26.2.build.129-stable'
     compileOnly 'org.geysermc.floodgate:api:2.2.4-SNAPSHOT'
     implementation 'org.apache.httpcomponents:httpclient:4.5.14'
     implementation 'com.sparkjava:spark-core:2.9.4'