RFR: 7903759: Amend changelog to include the removal of `jtdiff`

[Jonathan Gibbons]

On Thu, 15 Aug 2024 10:21:52 GMT, Christian Stein <cstein at openjdk.org> wrote:

> Please review this documentation-only PR updating the `CHANGELOG.md` file in preparation of the upcoming `jtreg` 7.5 release.

Marked as reviewed by jjg (Lead).

-------------

PR Review: https://git.openjdk.org/jtreg/pull/220#pullrequestreview-2240630543