feat: Upgrade to Spring Boot 3.2, and Virtual Threads if on Java 21

[SimonVerhoeven]

What's changed?

A new recipe to enable VT in case the upgrade is for a Java 21 project.

What's your motivation?

To facilitate the adaptation of Virtual Threads

Anything in particular you'd like reviewers to focus on?

There's no org.openrewrite.java.spring.boot3.UpgradeSpringBoot_3_2 recipe yet, but I wanted to keep that for a standalone PR.

Anyone you would like to review specifically?

Have you considered any alternatives or workarounds?

Any additional context

Checklist

• I've added unit tests to cover both positive and negative cases
• I've added the license header to any new files through ./gradlew licenseFormat
• I've used the IntelliJ IDEA auto-formatter on affected files

[timtebeek]

There's likely to be some conflicts with the PR I just created

• Generate property migrations for META-INF/spring-configuration-metadata.json too #445

We should indeed create a separate 3.2 migration recipe, and have that ready (and ideally documented) before next Thursday's release.

[SimonVerhoeven]

Seems likely, I'll keep an eye on that one and update this one based on that one.

[SimonVerhoeven]

The latest commit restores spring-boot-32.yml, and also adds the actual migration recipe.
This does mean it now also has a dependency on #447

[timtebeek]

Great work once again @SimonVerhoeven ! Thanks a lot for kicking off the Spring Boot 3.2 migration recipes & virtual thread adoption.