RFR: 8354078: Shenandoah: Make the generational mode be non-experimental (implementation)

[William Kemper]

Testing:

% ./build/linux-x86_64-server-fastdebug/jdk/bin/java -XX:+UseShenandoahGC -XX:ShenandoahGCMode=generational --version
openjdk 25 2025-09-16
OpenJDK Runtime Environment (fastdebug build 25-make-genshen-non-experimental)
OpenJDK 64-Bit Server VM (fastdebug build 25-make-genshen-non-experimental, mixed mode)

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Commit messages:
 - Make generational mode non-experimental

Changes: https://git.openjdk.org/jdk/pull/25270/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25270&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354078
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25270.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25270/head:pull/25270

PR: https://git.openjdk.org/jdk/pull/25270