RFR: 8355022: Implement JEP 506: Scoped Values [v4]

[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 incrementally with two additional commits since the last revision:

 - Remove unnecessary @since 25
 - Remove unnecessary @enablePreview in some tests

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24923/files
  - new: https://git.openjdk.org/jdk/pull/24923/files/d3f2ff1f..8f626617

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24923&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24923&range=02-03

  Stats: 4 lines in 4 files changed: 0 ins; 4 del; 0 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