RFR: 8344263: GenShen: Reduce extraneous log messages at INFO level [v5]

[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.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Update test, remove unused header, use proper case for old gen phases

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/540/files
  - new: https://git.openjdk.org/shenandoah/pull/540/files/1c7752c1..df0748ff

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=540&range=04
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=540&range=03-04

  Stats: 10 lines in 4 files changed: 1 ins; 2 del; 7 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