RFR: Make generational mode experimental

[William Kemper]

Setting `-XX:ShenandoahGCMode=generational` now requires `XX:+UnlockExperimentalVMOptions`.

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Commit messages:
 - Make generational experimental (now requires -XX:+UnlockExperimentalVMOptions)

Changes: https://git.openjdk.org/shenandoah/pull/283/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=283&range=00
  Stats: 7 lines in 4 files changed: 2 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/shenandoah/pull/283.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/283/head:pull/283

PR: https://git.openjdk.org/shenandoah/pull/283