Document Spring Boot 4 starter in README

[Copilot]

The Spring Boot 4 starter (db-scheduler-ui-spring-boot-4-starter) was added in v4.6.0 but not documented in the README.

Changes

• Prerequisites: Note Spring Boot 4.0 support alongside existing 3.3+ requirement
• Getting Started: Split dependency examples into Spring Boot 3 vs 4 variants with correct artifact IDs

<!-- Spring Boot 3 -->
<dependency>
    <groupId>no.bekk.db-scheduler-ui</groupId>
    <artifactId>db-scheduler-ui-starter</artifactId>
    <version>4.3.0</version>
</dependency>

<!-- Spring Boot 4 -->
<dependency>
    <groupId>no.bekk.db-scheduler-ui</groupId>
    <artifactId>db-scheduler-ui-spring-boot-4-starter</artifactId>
    <version>4.6.0</version>
</dependency>

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