Java 21 Language Features Support

Developers are enthusiastic about leveraging Java's latest language features, which offer more powerful, declarative, and expressive code; these features include unnamed variables, record patterns, and string templates. However, Checkstyle currently lacks robust support for these features. This project aims to bridge this gap by updating existing checks and potentially introducing new ones to ensure thorough coverage of Java 21 syntax and conventions proposed by the JEPs associated with these language advancements. The objective is to deliver comprehensive support for the new language features through revising check modules, exhaustive testing, and detailed documentation updates. This effort not only aligns Checkstyle with cutting-edge best practices in the Java community but also contributes to the project's ongoing evolution.