RFR: 8314755: Resource leak: SwingWorker listener keeps strong reference to executor

[Christopher Sahnwaldt]

In https://github.com/openjdk/jdk/commit/b8af3d50192f8bc98d83f8102f0fd1989f302e32 the weak reference was accidentally changed from a field to a local variable, which means that the PropertyChangeListener keeps a strong reference to executorService, which is a resource leak

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Commit messages:
 - SwingWorker.java: appContext shouldn't keep a strong reference to executorService

Changes: https://git.openjdk.org/jdk/pull/15081/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15081&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314755
  Stats: 4 lines in 1 file changed: 2 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15081.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15081/head:pull/15081

PR: https://git.openjdk.org/jdk/pull/15081