RFR: 8338766: [lw5] remove option enableNullRestrictedTypes and make null-restricted types a preview feature [v2]

[Vicente Romero]

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

Vicente Romero has updated the pull request incrementally with one additional commit since the last revision:

  ignore test RuntimeNullChecks

-------------

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1218/files
  - new: https://git.openjdk.org/valhalla/pull/1218/files/d3f1737e..37285f1a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1218&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1218&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 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