RFR: 8334315: Shenandoah: reduce GC logging noise

[Kelvin Nilsen]

Qualify certain log messages as ergo or debug to remove extraneous noise from regular gc logs.

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Commit messages:
 - Reduce verbosity of gc log messages
 - Revert "Make GC logging less verbose"
 - Make GC logging less verbose
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - ... and 9 more: https://git.openjdk.org/jdk/compare/8464ce6d...2a74ecc4

Changes: https://git.openjdk.org/jdk/pull/19795/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19795&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334315
  Stats: 17 lines in 1 file changed: 0 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/19795.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19795/head:pull/19795

PR: https://git.openjdk.org/jdk/pull/19795