RFR: 7903518: Release jtreg 7.3.1

[Christian Stein]

Please review this change to release jtreg 7.3.1 - the GIT tag `jtreg-7.3.1+1` will be added to the commit created by merging this PR.

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Commit messages:
 - Release jtreg 7.3.1

Changes: https://git.openjdk.org/jtreg/pull/168/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=168&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903518
  Stats: 6 lines in 2 files changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/168.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/168/head:pull/168

PR: https://git.openjdk.org/jtreg/pull/168