RFR: 7903460: Update version to 7.3

[Christian Stein]

The next promoted version of jtreg will be 7.3

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Commit messages:
 - 7903460: Update version to 7.3

Changes: https://git.openjdk.org/jtreg/pull/151/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=151&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903460
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/151.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/151/head:pull/151

PR: https://git.openjdk.org/jtreg/pull/151