RFR: 7903512: Update changelog for jtreg 7.3

Thu Jul 27 14:28:05 UTC 2023

On Wed, 26 Jul 2023 15:27:23 GMT, Christian Stein <cstein at openjdk.org> wrote:

> 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.

Marked as reviewed by jjg (Lead).

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PR Review: https://git.openjdk.org/jtreg/pull/162#pullrequestreview-1549984900