Remove 'minimumRuntime' source set from buildSrc

[mark-vieira]

We currently separate out some core classes in buildSrc into the minimumRuntime source set. This source set is compiled with a target bytecode version of Java 8 for compatibility with builds unable to migration forward to Java 11, primarily elasticsearch-hadoop. They have since been able to bump their Gradle and JDK versions so we can get rid of this complexity and fold this code back into the main sourceset and use the same compiler version as everything else.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Build)

[mark-vieira]

@breskeby do you mind taking a look at this when you find some time?

[breskeby]

👍 will do