RFR: 8372534: Update Libpng to 1.6.51

[Jayathirth D V]

Upgrade libpng used for Splashscreen from 1.6.47 to 1.6.51 version.

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

Commit messages:
 - Update
 - update .md file
 - 8372534: Update Libpng to 1.6.51

Changes: https://git.openjdk.org/jdk/pull/28518/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28518&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372534
  Stats: 638 lines in 21 files changed: 297 ins; 207 del; 134 mod
  Patch: https://git.openjdk.org/jdk/pull/28518.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28518/head:pull/28518

PR: https://git.openjdk.org/jdk/pull/28518