RFR: 8369983: Remove expired ZGC flags for JDK 26

[Joel Sikström]

Hello,

The ZGenerational flag was obsoleted in JDK 24 as part of removing the non-generational mode of ZGC in JEP 490. The flag was still obsolete in JDK 25, which is a LTS version, so that user's that might specify/try to use it are prompted with a warning. Now that we're past JDK 25, the flag should be expired. Likewise, the ZMarkStackSpaceLimit flag was obsoleted in JDK 25 and can now be expired in JDK 26.

Since neither of these flags were documented in `java.md` I haven't added them to the "Removed Java Options" section in that file.

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Commit messages:
 - 8369983: Remove expired ZGC flags for JDK 26

Changes: https://git.openjdk.org/jdk/pull/27838/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27838&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369983
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27838.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27838/head:pull/27838

PR: https://git.openjdk.org/jdk/pull/27838