[#2491] Add spring-boot-4-starter module

[cafec0c0]

This PR addresses the issue raised in #2491 which requests support for Spring Boot 4.

To accommodate this, a new spring-boot-4-starter module has been added.

The new module reuses the existing spring boot code, as it is still code-compatible with the code for spring boot 2 and 3, but does require using newer spring boot dependencies (4.x.x).

This unfortunately requires a Java 17+ compiler, and Java 9+ drops support for targeting Java 1.5.

I've thought of two solutions to this, but any others are welcomed:

1. 2 Compiler passes: the first with a Java 8 compiler and the second with a Java 17 compiler. I think this would need some CI configuration to only use Java 17 for modules that require it (i.e. running ./gradlew :picocli-spring-boot-4-starter:build), so we don't inadvertently overwrite the modules compiled previously.
2. Use Java 17+ for all modules: this would simplify CI, but would break Java 1.5 and 1.6 compatibility for modules.