RFR: 8289616: Drop use of Thread.stop in AppContext

[Phil Race]

Thread.stop() and ThreadGroup.destroy() are called by sun.awt.AppContext.dispose()

These should no longer be called. Both are deprecated for removal.
ThreadGroup.destroy() does nothing. Thread.stop() throws UnsupportedOperationException
for virtual threads, and is expected to do so for all threads if it is not removed before then.

This fix stops calling these methods and prints a warning from dispose() to encourage
callers to move away from AppContext which itself will likely be removed in a future release
since the Webstart and Plugin scenarios that needed it are no longer supported.
Although most tests that use AppContext are now irrelevant only one test actually fails
and is removed  so we are doing the least here and the big job of completely getting rid of
AppContext is for another day.

-------------

Commit messages:
 - 8289616

Changes: https://git.openjdk.org/jdk/pull/9863/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9863&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289616
  Stats: 118 lines in 2 files changed: 7 ins; 111 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9863.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9863/head:pull/9863

PR: https://git.openjdk.org/jdk/pull/9863