RFR: 8338766: [lw5] remove option enableNullRestrictedTypes and make null-restricted types a preview feature

[Vicente Romero]

this fix is removing internal option `enableNullRestrictedTypes` and making null restricted types for value classes a preview feature

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Commit messages:
 - 8338766: [lw5] remove option enableNullRestrictedTypes and make null-restricted types a preview feature

Changes: https://git.openjdk.org/valhalla/pull/1218/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1218&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338766
  Stats: 195 lines in 29 files changed: 118 ins; 4 del; 73 mod
  Patch: https://git.openjdk.org/valhalla/pull/1218.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1218/head:pull/1218

PR: https://git.openjdk.org/valhalla/pull/1218