RFR: 8323779: Serial: Remove Generation::promotion_attempt_is_safe

[Albert Mingkun Yang]

Simple updating a field type to use more concrete type.

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Commit messages:
 - s1-remove-promotion

Changes: https://git.openjdk.org/jdk/pull/17439/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17439&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323779
  Stats: 22 lines in 5 files changed: 5 ins; 14 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17439.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17439/head:pull/17439

PR: https://git.openjdk.org/jdk/pull/17439