Fix JDK 11.0.2 javadoc (#8276)
Fix JDK 11.0.2 javadoc (#8276)

JDK 11.0.2 has been released. However, gradle `javadocAll` task fails on 11.0.2.

This seems to be caused by https://bugs.openjdk.java.net/browse/JDK-8212233 , when the code being documented uses modules, JDK 11.0.2's `javadoc` expects all its references use modules. Apparently this can't be true for Java 8: https://github.com/gradle/gradle/blob/df7ac484a2facbb4503ba911666367de22b1d16a/subprojects/docs/docs.gradle#L296

Now we change reference link to JDK 11 API: `def javaApiUrl = "https://docs.oracle.com/en/java/javase/11/docs/api"`. This would make all links to point to Java 11 API page.
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