MINOR: Work around OpenJDK 11 javadocs issue. (#6747)
MINOR: Work around OpenJDK 11 javadocs issue. (#6747)

Some versions of OpenJDK 11 do not properly handle external javadocs links referencing previous Java versions. See: https://bugs.openjdk.java.net/browse/JDK-8212233.

Failure symptom:
`> Task :connect:api:javadoc
javadoc: error - The code being documented uses modules but the packages defined in https://docs.oracle.com/javase/8/docs/api/ are in the unnamed module.
1 error`

This PR conditionally sets the Java api docs link for the affected Gradle tasks. I verified that the links render correctly in the generated documentation when building with `1.8.0_181` and `11.0.3`. For example, in `build/docs/javadoc/org/apache/kafka/connect/source/SourceTask.html` the hyperlink to `java.nio.channels.Selector` points to a valid page on Oracle's site in both cases.

Reviewers: José Armando García Sancio <a8f36d709c55b131453e2fdebab89d11b4e5c5e0@users.noreply.github.com>, Jason Gustafson <jason@confluent.io>
TiVo/kafka,guozhangwang/kafka,TiVo/kafka,noslowerdna/kafka,apache/kafka,guozhangwang/kafka,guozhangwang/kafka,sslavic/kafka,KevinLiLu/kafka,guozhangwang/kafka,noslowerdna/kafka,apache/kafka,lindong28/kafka,noslowerdna/kafka,Chasego/kafka,apache/kafka,sslavic/kafka,KevinLiLu/kafka,TiVo/kafka,lindong28/kafka,TiVo/kafka,Chasego/kafka,lindong28/kafka,KevinLiLu/kafka,sslavic/kafka,apache/kafka,Chasego/kafka,lindong28/kafka,KevinLiLu/kafka,Chasego/kafka,noslowerdna/kafka,sslavic/kafka