RFR: 8310335 JFR: Modernize collections and switch statements [v3]

[Erik Gahlin]

> Could I have a review of an update to the jfr modules to use more recent APIs and language constructs.
> 
> Testing: test/jdk/jdk/jfr
> 
> Thanks
> Erik

Erik Gahlin has updated the pull request incrementally with one additional commit since the last revision:

  Unmodernize TestAgentEvent.java. Will be handled seperately, suitable for JDK 21 backport

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14547/files
  - new: https://git.openjdk.org/jdk/pull/14547/files/c0f59696..fb668c00

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14547&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14547&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14547.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14547/head:pull/14547

PR: https://git.openjdk.org/jdk/pull/14547