RFR: CODETOOLS-7903332: Update version of jtharness to 6+24

[Jonathan Gibbons]

Please review a small update to have jtreg use JT Harness 6+24.

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Commit messages:
 - CODETOOLS-7903332: Update version of jtharness to 6+24

Changes: https://git.openjdk.org/jtreg/pull/130/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=130&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903332
  Stats: 8 lines in 4 files changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jtreg/pull/130.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/130/head:pull/130

PR: https://git.openjdk.org/jtreg/pull/130