Outdated project import and build instructions in CONTRIBUTING.adoc #3375

Describe the bug

The setup instructions in CONTRIBUTING.adoc do not match the current repository structure and build configuration:

1. The IntelliJ IDEA instructions say to open the root build.gradle, but the repository uses Maven and has a root pom.xml.
2. The build command is listed as ./mvn install, but there is no mvn script in the repository root. The CI workflow invokes the installed mvn command.
3. The guide says the source can be built with JDK 1.8 or above. This guidance is outdated for the current Spring Boot 4.x project. The CI workflow uses JDK 21.

To Reproduce

Compare the following instructions with the tracked files and CI configuration on main at commit 354c98cbab303131573c47a4aa39ded8c579dbb3:

• IntelliJ IDEA import instructions
• Building from source
• Repository root
• CI workflow

Expected behavior

The contribution guide should:

• Point contributors to the root pom.xml when importing the project.
• Use mvn install as the build command and mention that Maven must be installed.
• Update the Java setup guidance for the current branch, distinguishing the minimum requirement from JDK 21, which is used by CI.

Additional context

These findings are based on inspecting the repository files and CI configuration.