RFR: 7903943: Release jtreg 7.5.1 [v2]

[Christian Stein]

> Please review this change preparing the `7.5.1` release of `jtreg`.
> 
> The version number `7.6` will be restored on the main branch after the release; a dedidated `7.5.x` branch was not required, yet.
> 
> This change also updates the `CHANGELOG.md` file with noteworthy items. It also removes the detailed description of the now removed `LIBRARY.properties` support from the `7.5` section.

Christian Stein has updated the pull request incrementally with two additional commits since the last revision:

 - Update Gradle Wrapper validation action
 - Update copyright year

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

Changes:
  - all: https://git.openjdk.org/jtreg/pull/245/files
  - new: https://git.openjdk.org/jtreg/pull/245/files/1703504c..c1c5b3ae

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=245&range=01
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=245&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/245.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/245/head:pull/245

PR: https://git.openjdk.org/jtreg/pull/245