SpringBoot Remove sourceCompatibility/targetCompatibility blocks from Gradle plugin integration tests

A lot have this:

java {
    sourceCompatibility = '1.8'
    targetCompatibility = '1.8'
}

Comment From: wilkinsona

I'm not sure that they should be removed. In 3.0.x and later, their values should probably be changed to 17 instead.

Comment From: scottfrederick

It turns out a small minority of Gradle integration test build files have the compatibility blocks, and the test run fine without them, so we'll remove them all.