Use JDK 10 for 6.4 BWC builds

[jasontedor]

The 6.4 release will officially be built with JDK 10. We should push JDK 10 down to the BWC builds for the 6.4 branch then too.

[elasticmachine]

Pinging @elastic/es-core-infra

[jasontedor]

This is not ideal from a maintenance perspective, but I think the simplest way is to maintain it like this as we do not know until we cut a branch what JDK we will use for the official product builds. I think we can add to our internal docs that this needs to be updated when we cut a new stable branch?

[alpar-t]

I think this is another example in favor of having a master meta info file as described in #32128 to have all this information in a single place and consumed throughout the build.

[jasontedor]

I agree @atorok, see also

elasticsearch/buildSrc/src/main/groovy/org/elasticsearch/gradle/test/NodeInfo.groovy

Lines 176 to 180 in 7c0fc20

	if (nodeVersion.before("6.2.0")) {
	javaVersion = 8
	} else if (nodeVersion.onOrAfter("6.2.0") && nodeVersion.before("6.3.0")) {
	javaVersion = 9
	}

for a similar need.