RFR: 7903513: Update version to 7.4

[Christian Stein]

Update jtreg's version to 7.4

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Commit messages:
 - 7903513: Update version to 7.4

Changes: https://git.openjdk.org/jtreg/pull/163/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=163&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903513
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/163.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/163/head:pull/163

PR: https://git.openjdk.org/jtreg/pull/163