Bundle JDK 14 in the 6.8 Docker images

[jasontedor]

JDK 14 will be released on March 17, 2020, at which point JDK 13 will be end of life. We should upgrade the JDK bundled in the 6.8 Docker images:

elasticsearch/distribution/docker/build.gradle

Lines 26 to 27 in 01af65d

	'jdkUrl' : 'https://github.com/AdoptOpenJDK/openjdk13-binaries/releases/download/jdk-13.0.2%2B8/OpenJDK13U-jdk_x64_linux_hotspot_13.0.2_8.tar.gz',
	'jdkVersion' : '13.0.2+8',

See #51511 for a similar change.

[elasticmachine]

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