git: openjdk/jtreg: Update version to 6.0

[Jonathan Gibbons]

Changeset: fd8cea06
Author:    Jonathan Gibbons <jonathan.gibbons at oracle.com>
Date:      2021-01-22 15:31:41 +0000
URL:       https://git.openjdk.java.net/jtreg/commit/fd8cea06

Update version to 6.0

! make/build-support/version-numbers