RFR: 7903512: Update changelog for jtreg 7.3

[Christian Stein]

Please review this update to the changelog for jtreg 7.3.

This commit also tidies up the **display names** of the version headers. From `7.1.1+1` to a more human-readable form of `7.1.1`. The underlying tag names are kept as-is.

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Commit messages:
 - 7903512: Update changelog for jtreg 7.3

Changes: https://git.openjdk.org/jtreg/pull/162/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=162&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903512
  Stats: 34 lines in 1 file changed: 16 ins; 8 del; 10 mod
  Patch: https://git.openjdk.org/jtreg/pull/162.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/162/head:pull/162

PR: https://git.openjdk.org/jtreg/pull/162