RFR: 7904063: Update version to 8.1

[Iris Clark]

On Fri, 11 Jul 2025 09:27:37 GMT, Christian Stein <cstein at openjdk.org> wrote:

> Please review this trivial change starting the development of jtreg 8.1
> 
> This PR also fixes comparison links in `CHANGELOG.md` by using the correct build tag: `jtreg-8+2`

Marked as reviewed by iris (Reviewer).

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PR Review: https://git.openjdk.org/jtreg/pull/274#pullrequestreview-3013978427