Support building with modern JDKs while keeping Java 8 runtime compatibility

The bundled Java externals and the core Java build assumed old JDKs and
emitted pre-8 bytecode at pre-8 source/target lev

Support building with modern JDKs while keeping Java 8 runtime compatibility

The bundled Java externals and the core Java build assumed old JDKs and
emitted pre-8 bytecode at pre-8 source/target levels. Modern JDKs (11+)
reject -source/-target 1.4/1.5/6/7 outright, and even where compilation
succeeds a new JDK silently emits bytecode that will not load on the
minimum supported Java 8 runtime.

Establish a tree-wide Java 8 bytecode floor and fix the bundled externals
so the tree builds on any JDK from 8 through 21 while all artifacts remain
loadable on a Java 8 JRE.

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