deps: bump net.kyori:adventure-text-serializer-plain (#503)

Bumps [net.kyori:adventure-text-serializer-plain](https://github.com/KyoriPowered/adventure) from 4.20.0 to 4.21.0.
- [Release notes](https://github.com/KyoriPowered/adventure/releases)
- [Commits](https://github.com/KyoriPowered/adventure/compare/v4.20.0...v4.21.0)

---
updated-dependencies:
- dependency-name: net.kyori:adventure-text-serializer-plain
  dependency-version: 4.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

common/build.gradle

@@ -25,7 +25,7 @@ dependencies {
     compileOnly 'org.jetbrains:annotations:26.0.2'
     compileOnly 'net.kyori:adventure-api:4.20.0'
     compileOnly 'net.kyori:adventure-platform-api:4.3.4'
-    compileOnly "net.kyori:adventure-text-serializer-plain:4.20.0"
+    compileOnly "net.kyori:adventure-text-serializer-plain:4.21.0"
     compileOnly 'com.google.guava:guava:33.4.8-jre'
     compileOnly 'com.github.plan-player-analytics:Plan:5.5.2272'
     compileOnly "redis.clients:jedis:$jedis_version"