[issue #4175] SpringBootExplodedProcessor now uses JAR's Main-Class w…

[stefan-prange]

…hen creating the target image's entrypoint

Thank you for your interest in contributing! For general guidelines, please refer to the contributing guide.

Before filing a pull request, make sure to do the following:

• [X] Create a new issue at https://github.com/GoogleContainerTools/jib/issues/new/choose. -> Done: see https://github.com/GoogleContainerTools/jib/issues/4175
• [ ] Ensure that your implementation plan is approved by the team. -> I hope so. Bugfixing is always a good idea, isn't it?
• [ ] Verify that integration tests and unit tests are passing after the change. -> All unit tests passed, but one of the integration tests failed on my machine (WarCommandTest). That's not related to my code additions because running the integration test on master came to the same result.
• [X] Address all checkstyle issues. Refer to the style guide. -> I ran the auto-formatter, so I hope my additions comply with your styling preferences.

This helps to reduce the chance of having a pull request rejected.

Fixes #4175 🛠️