Support spring boot 3 on decaton-spring

[koyapei]

motivation

There has been a change in the auto-configuration file in Spring Boot 3.
https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-3.0-Migration-Guide#auto-configuration-files

In the decaton-spring module, it is necessary to support both boot 2 and boot 3 until migrating to boot 3.

modification

• In addition to the spring.factories used in boot 2, a new configuration file spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports was added for boot 3 in META-INF.
• Integration tests have been created to verify that auto-configuration works in both boot 2 and boot 3.
  • If these tests are excessive, I'll remove them.

[kawamuray]

https://spring.io/projects/spring-boot#support

spring boot 2 will soon reach EOL, so it would be okay to just drop spring boot2 support in this PR?

[koyapei]

spring boot 3 requires Java 17 or higher, so if we discontinue support for spring boot 2, we also need to discontinue support for Java 16 or lower at the same time.
https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-3.0-Migration-Guide#review-system-requirements

Although it is possible to discontinue support for them in decaton-spring, it is not highly desirable as it would compromise consistency of java's version with other modules.

[kawamuray]

I see. Sounds like a good comprise then.

[kawamuray]

LGTM, thanks!

[koyapei]

Thanks!