[jfx21] RFR: 8312528: Move Subscription interface from javafx.beans to javafx.util

[John Hendrikx]

Backport of https://bugs.openjdk.org/browse/JDK-8312529

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Commit messages:
 - Backport cc57fb51a9409f5d2a6d2ce8dcd3878d48f9cfc9

Changes: https://git.openjdk.org/jfx/pull/1185/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1185&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312528
  Stats: 20 lines in 12 files changed: 9 ins; 7 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1185.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1185/head:pull/1185

PR: https://git.openjdk.org/jfx/pull/1185