RFR: JDK-8316030: Update Libpng to 1.6.40

[Harshitha Onkar]

Libpng updated from 1.6.39 to 1.6.40

Build and CI Testing passes on all platforms.

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Commit messages:
 - .md file changes
 - libpng source file changes

Changes: https://git.openjdk.org/jdk/pull/16223/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16223&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316030
  Stats: 246 lines in 11 files changed: 89 ins; 82 del; 75 mod
  Patch: https://git.openjdk.org/jdk/pull/16223.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16223/head:pull/16223

PR: https://git.openjdk.org/jdk/pull/16223