Address PR feedback: Java 21 requirement and cleanup

- Update Java toolchain from 17 to 21 (Spring Boot 4 requirement)
- Remove unused springBootVersion property from ext block
- Add comment clarifying thymeleaf-extras-springsecurity6 compatibility
  with Spring Security 7 (no springsecurity7 artifact exists yet)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: devondragon

build.gradle

@@ -17,13 +17,12 @@ group = 'com.digitalsanctuary'
 description = 'Spring User Framework'
 
 ext {
-    springBootVersion = '3.5.5'
     lombokVersion = '1.18.42'
 }
 
 java {
     toolchain {
-        languageVersion = JavaLanguageVersion.of(17)
+        languageVersion = JavaLanguageVersion.of(21)
     }
 }
 
@@ -41,6 +40,8 @@ dependencies {
     compileOnly 'org.springframework.boot:spring-boot-starter-security'
     compileOnly 'org.springframework.boot:spring-boot-starter-thymeleaf'
     compileOnly 'org.springframework.boot:spring-boot-starter-web'
+    // Note: thymeleaf-extras-springsecurity6 is compatible with Spring Security 7
+    // No springsecurity7 artifact exists yet - Thymeleaf team uses springsecurity6 for 6.x+
     compileOnly 'org.thymeleaf.extras:thymeleaf-extras-springsecurity6:3.1.3.RELEASE'
     compileOnly 'nz.net.ultraq.thymeleaf:thymeleaf-layout-dialect:3.4.0'