RFR: 8283218: Update GStreamer to 1.20.1

[Alexander Matveev]

- GStreamer updated to 1.20.1 and GLib updated to 2.72.0.
- No changes to our code, except in GstAudioSpectrum.cpp g_atomic_pointer_compare_and_exchange() was changed to g_atomic_pointer_set(). For some reason I was not able to get code compiled with g_atomic_pointer_compare_and_exchange() used from C++ code. Also, I do not see a need to use g_atomic_pointer_compare_and_exchange(), since m_pHolder always equals to old_holder.
- Tested on all platforms with all supported media streams.

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Commit messages:
 - 8283218: Update GStreamer to 1.20.1

Changes: https://git.openjdk.java.net/jfx/pull/768/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=768&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283218
  Stats: 38793 lines in 410 files changed: 21828 ins; 6021 del; 10944 mod
  Patch: https://git.openjdk.java.net/jfx/pull/768.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/768/head:pull/768

PR: https://git.openjdk.java.net/jfx/pull/768