RFR: 8355022: Implement JEP 506: Scoped Values [v6]

[Andrew Haley]

> Propose to finalize scoped values.
> The only functional change is that the orElse() method no longer accepts a null argument.

Andrew Haley has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains nine commits:

 - Fix merge
 - Merge from https://github.com/openjdk/jdk
 - ScopedValue::orElse() does not accept null as an argument.
 - Remove unnecessary @since 25
 - Remove unnecessary @enablePreview in some tests
 - Since when?
 - ScopedValue::orElse() does not accept null as an argument.
 - 8355720: Implement JEP 506: Scoped Values
 - Implement JEP 506: Scoped Values

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Changes: https://git.openjdk.org/jdk/pull/24923/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24923&range=05
  Stats: 21 lines in 5 files changed: 3 ins; 11 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/24923.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24923/head:pull/24923

PR: https://git.openjdk.org/jdk/pull/24923