Upgrade to newer spring boot version in test suite

The current `spring-boot-gradle-plugin` version (2.2.11.RELEASE) used in the unit test suite is pretty old. Suppressed the major version bump in dependabot #3876, but we should try manually upgrading to a newer 2.x version. I think this was previously blocked on the gradle build version, which has been updated since.

Currently the [unit tests](https://github.com/GoogleContainerTools/jib/blob/master/jib-gradle-plugin/src/test/java/com/google/cloud/tools/jib/gradle/JibPluginTest.java) cover different conditions from #2284 to address `jar` and `bootJar` task output location conflicts. As of Spring Boot 2.5.0, the [packaging default behaviors](https://docs.spring.io/spring-boot/docs/2.5.0/gradle-plugin/reference/htmlsingle/#packaging-executable.and-plain-archives) for jar and war tasks have changed. Will also need to look into this change as part of the manual upgrade.