SpringBoot Reduce Gradle compatibility tests to oldest and newest minor of each major

In 2.7.x we currently test again 6.8.3, 6.9.3, 7.0.2, 7.1.1, 7.2, 7.3.3, 7.4.2, 7.5.1. This can consume a considerable amount of time, both for local builds and on CI. We don't recall ever finding a version-specific Gradle bug so we think we've got the balance wrong here. We're going to reduce the versions that we test against to the old and newest of each supported major. That will reduce the 8 versions listed previously to four: 6.8.3, 6.9.3, 7.0.2, and 7.5.1.