[lworld] RFR: 8328543: [lworld] Library and test updates to use --enable-preview instead of ValhallaFeatures

[Roger Riggs]

Update AccessFlag and tests to JEP 401 and use of --enable-preview instead of ValhallaFeatures.
Minor formatting change to java.lang.Character to allow it to be a value class with --enable-preview.
(The class file format version numbers will need to be updated when re-synching with the mainline).

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Commit messages:
 - 8328543: [lworld] Library and test updates to use --enable-preview instead of ValhallaFeatures

Changes: https://git.openjdk.org/valhalla/pull/1060/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1060&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328543
  Stats: 49 lines in 7 files changed: 10 ins; 17 del; 22 mod
  Patch: https://git.openjdk.org/valhalla/pull/1060.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1060/head:pull/1060

PR: https://git.openjdk.org/valhalla/pull/1060