Update Java toolchain version from 17 to 21

Updated the build.gradle to use Java 21 instead of Java 17 to match
the available JDK in the build environment. This allows the project to
build successfully.

Author: claude

build.gradle

@@ -24,7 +24,7 @@ description = 'John Q Walker Node Positioning Algorithm'
 
 java {
     toolchain {
-        languageVersion = JavaLanguageVersion.of(17)
+        languageVersion = JavaLanguageVersion.of(21)
     }
 }