RFR: 8308335 JFR: Remove @Experimental from Virtual Threads events

[Erik Gahlin]

Virtual threads is no longer in preview and experimental annotation should be removed.

Testing: test/jdk/jdk/jfr

Thanks
erik

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Commit messages:
 - Initial

Changes: https://git.openjdk.org/jdk/pull/14133/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14133&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308335
  Stats: 18 lines in 5 files changed: 4 ins; 12 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14133.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14133/head:pull/14133

PR: https://git.openjdk.org/jdk/pull/14133