RFR: 8331081: 'internal proprietary API' diagnostics if --system is configured to an earlier JDK version

[Liam Miller-Cushon]

This change fixes a bug preventing javac from emitting 'compiler.warn.sun.proprietary' diagnostics if `--system` is set to a non-default value. The diagnostics are currently emitted for values of `--release`, and for the default value of `--system`.

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Commit messages:
 - 8331081: 'internal proprietary API' diagnostics if --system is configured to an earlier JDK version

Changes: https://git.openjdk.org/jdk/pull/19069/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19069&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331081
  Stats: 129 lines in 2 files changed: 127 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19069.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19069/head:pull/19069

PR: https://git.openjdk.org/jdk/pull/19069