RFR: 8344263: GenShen: Reduce extraneous log messages at INFO level

[William Kemper]

Genshen logs a lot of fine level detail at `gc=info`. This category should be reserved for major phase changes in the gc cycle.

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Commit messages:
 - Reduce logging at info, use proper case for generation names

Changes: https://git.openjdk.org/shenandoah/pull/540/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=540&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344263
  Stats: 58 lines in 10 files changed: 0 ins; 1 del; 57 mod
  Patch: https://git.openjdk.org/shenandoah/pull/540.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/540/head:pull/540

PR: https://git.openjdk.org/shenandoah/pull/540