RFR: 8347620: Shenandoah: Use 'free' tag for free set related logging

[William Kemper]

Without a distinguishing tag, debug logging is too voluminous to enable when we really only want the free set's debug messages.

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Commit messages:
 - Use 'free' tag with free set messages

Changes: https://git.openjdk.org/jdk/pull/23086/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23086&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347620
  Stats: 78 lines in 1 file changed: 7 ins; 7 del; 64 mod
  Patch: https://git.openjdk.org/jdk/pull/23086.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23086/head:pull/23086

PR: https://git.openjdk.org/jdk/pull/23086