Update project for Java 21

[mrMigles]

Summary

• configure Maven build to target Java 21 via updated compiler settings
• switch Docker base image to a Java 21 runtime
• tidy build script output formatting

Testing

• java -version
• ./mvnw -version (fails: network is unreachable while downloading Maven wrapper distribution)

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Add javax.annotation-api for Java 21 build

Switching the compiler target to Java 21 (maven-compiler-plugin release 21) means the build now runs on a JDK that no longer ships javax.annotation (removed with the old Java EE modules). The codebase still imports javax.annotation.PostConstruct/Nonnull in multiple classes (e.g., src/main/java/ru/holyway/botplatform/BotPlatformApplication.java:13), and the POM does not declare a javax.annotation-api dependency, so compilation on Java 21 will fail with “package javax.annotation does not exist.” Please add the API dependency or keep targeting a JDK that provides it.

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