RFR: 8318894: G1: Use uint for age in G1SurvRateGroup

[Albert Mingkun Yang]

Simple refactoring to use unsigned type for region-age.

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Commit messages:
 - g1-age-uint

Changes: https://git.openjdk.org/jdk/pull/16376/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16376&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318894
  Stats: 41 lines in 5 files changed: 6 ins; 10 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/16376.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16376/head:pull/16376

PR: https://git.openjdk.org/jdk/pull/16376