RFR: 7903944: Update version to 7.6

[Christian Stein]

Please review this trivial change updating the version to `7.6`; the upcoming release version of `jtreg`.

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Commit messages:
 - 7903944: Update version to 7.6

Changes: https://git.openjdk.org/jtreg/pull/246/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=246&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903944
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/246.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/246/head:pull/246

PR: https://git.openjdk.org/jtreg/pull/246