RFR: 8334315: Shenandoah: reduce GC logging noise

[William Kemper]

Not clean, just logging level changes.

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Commit messages:
 - Backport f35af7171213c09107b99104a73022853bff91b0

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/109/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=109&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334315
  Stats: 22 lines in 1 file changed: 0 ins; 0 del; 22 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/109.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/109/head:pull/109

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/109