Update README for Spring Boot 4.0 and add version compatibility

- Update badges: Java 21+, Spring Boot 4.0
- Add Version Compatibility section with table
- Document v1.0.0-springboot3 tag for Spring Boot 3.x users
- Update prerequisites to require Java 21
- Update Technology Stack table (Spring Boot 4.0+, Spring Security 7)
- Update troubleshooting to reference Java 21

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

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

Author: devondragon

README.md

@@ -1,8 +1,8 @@
 # Spring User Framework Demo Application
 
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
-[![Java Version](https://img.shields.io/badge/Java-17%2B-brightgreen)](https://www.oracle.com/java/technologies/javase/jdk17-archive-downloads.html)
-[![Spring Boot](https://img.shields.io/badge/Spring%20Boot-3.5.6-green)](https://spring.io/projects/spring-boot)
+[![Java Version](https://img.shields.io/badge/Java-21%2B-brightgreen)](https://www.oracle.com/java/technologies/downloads/)
+[![Spring Boot](https://img.shields.io/badge/Spring%20Boot-4.0-green)](https://spring.io/projects/spring-boot)
 [![Gradle](https://img.shields.io/badge/Gradle-8.0%2B-blue)](https://gradle.org/)
 [![Docker](https://img.shields.io/badge/Docker-Supported-blue)](https://www.docker.com/)
 [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](contributing)
@@ -14,6 +14,7 @@ A comprehensive demonstration application for the [Spring User Framework](https:
 
 ## Table of Contents
 - [Overview](#overview)
+- [Version Compatibility](#version-compatibility)
 - [Features](#features)
 - [Prerequisites](#prerequisites)
 - [Quick Start](#quick-start)
@@ -35,6 +36,30 @@ This demo application serves as a reference implementation of the [Spring User F
 
 The application implements an event management system where users can browse, register for, and manage events. This demonstrates how to build application-specific functionality on top of the user management framework.
 
+## Version Compatibility
+
+This demo application tracks the latest version of the Spring User Framework:
+
+| Demo App Version | Spring Boot | Spring User Framework | Java | Branch/Tag |
+|------------------|-------------|----------------------|------|------------|
+| main (current) | 4.0.x | 4.0.x | 21+ | `main` |
+| 1.0.0-springboot3 | 3.5.x | 3.5.x | 17+ | [`v1.0.0-springboot3`](https://github.com/devondragon/SpringUserFrameworkDemoApp/tree/v1.0.0-springboot3) |
+
+### Using Spring Boot 3.x?
+
+If you need to use Spring Boot 3.5.x with Java 17, use the tagged version:
+
+```bash
+git clone https://github.com/devondragon/SpringUserFrameworkDemoApp.git
+cd SpringUserFrameworkDemoApp
+git checkout v1.0.0-springboot3
+```
+
+This version uses:
+- Spring Boot 3.5.6
+- Spring User Framework 3.5.1
+- Java 17+
+
 ## Features
 
 - **User Management**
@@ -68,7 +93,7 @@ The application implements an event management system where users can browse, re
 
 Before you begin, ensure you have the following installed:
 
-- **Java**: JDK 17 or higher ([Download](https://www.oracle.com/java/technologies/javase/jdk17-archive-downloads.html))
+- **Java**: JDK 21 or higher ([Download](https://www.oracle.com/java/technologies/downloads/)) - *Note: For Spring Boot 3.x version, Java 17+ is sufficient*
 - **Database**: MariaDB, MySQL, or Docker for containerized database
 - **Build Tool**: Gradle (included via wrapper) or Maven
 - **Optional**: Docker and Docker Compose for containerized setup
@@ -571,8 +596,8 @@ This project supports **Spring Boot DevTools** for live reload and auto-restart.
 | Layer          | Technology                  | Purpose                                        |
 | -------------- | --------------------------- | ---------------------------------------------- |
 | **Frontend**   | Thymeleaf + Bootstrap       | Server-side rendering with responsive UI       |
-| **Backend**    | Spring Boot 3.4+            | Application framework and dependency injection |
-| **Security**   | Spring Security             | Authentication, authorization, CSRF protection |
+| **Backend**    | Spring Boot 4.0+            | Application framework and dependency injection |
+| **Security**   | Spring Security 7           | Authentication, authorization, CSRF protection |
 | **Data**       | Spring Data JPA + Hibernate | Object-relational mapping and data access      |
 | **Database**   | MariaDB/MySQL               | Primary data persistence                       |
 | **Testing**    | JUnit 5 + Selenide          | Unit, integration, and UI testing              |
@@ -601,7 +626,7 @@ Solution:
 Solution:
 1. ./gradlew clean build --refresh-dependencies
 2. Check internet connection
-3. Verify Java version: java -version (requires JDK 17+)
+3. Verify Java version: java -version (requires JDK 21+)
 4. Clear Gradle cache: rm -rf ~/.gradle/caches
 ```