RFR: 8354091: Update RELEASE_25 description for Flexible Constructor Bodies

[Joe Darcy]

Getting ready for this SourceVersions change to be pushed after 

    JDK-8344703: Compiler Implementation for Flexible Constructor Bodies
    
is integrated.

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Commit messages:
 - JDK-8354091: Update RELEASE_25 description for Flexible Constructor Bodies

Changes: https://git.openjdk.org/jdk/pull/25053/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25053&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354091
  Stats: 7 lines in 1 file changed: 3 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/25053.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25053/head:pull/25053

PR: https://git.openjdk.org/jdk/pull/25053