Add Check Support for Java 21 Unnamed Variables & Patterns Syntax: InnerAssignment

child of #14942:  

check doc: https://checkstyle.org/checks/coding/innerassignment.html

---

```
public class Test {
    void test(Object obj) {
       boolean _ = false;
       if ( _ = true) {} // compile time error, Using '_' as a reference is not allowed
    }
}

```
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Unnamed variables can't be assigned. no update is needed