git: openjdk/jtreg: revert TestNG back to 7.3.0

[Jonathan Gibbons]

Changeset: b98cff11
Author:    Jonathan Gibbons <jonathan.gibbons at oracle.com>
Date:      2021-05-11 09:13:24 +0000
URL:       https://git.openjdk.java.net/jtreg/commit/b98cff115d4bd3888a2243faccf3725c42db9baa

revert TestNG back to 7.3.0
minor updates to README.md and build.sh

! README.md
! make/build-support/version-numbers
! make/build.sh