[jdk8u] RFR: 8372534: Update Libpng to 1.6.51

[Sergey Bylokhov]

On Sat, 27 Dec 2025 18:26:18 GMT, Andrew John Hughes <andrew at openjdk.org> wrote:

> Update to the latest version of libpng. Change includes both the backport of the latest libpng change, [JDK-8372534](https://bugs.openjdk.org/browse/JDK-8372534), and previous updates not present in 8u ([JDK-8316030](https://bugs.openjdk.org/browse/JDK-8316030) - 1.6.40, [JDK-8329004](https://bugs.openjdk.org/browse/JDK-8329004) - 1.6.43, [JDK-8348598](https://bugs.openjdk.org/browse/JDK-8348598) - 1.6.47). The change built fine with GCC 8 on RHEL 8 and `java/awt/SplashScreen` tests passed.
> 
> I also included `UPDATING.txt` introduced in [JDK-8208353](https://bugs.openjdk.org/browse/JDK-8208353) in 11u, but not 8u, and updated in [JDK-8295685](https://bugs.openjdk.org/browse/JDK-8295685) as part of the JDK-8329004 backport which updates it.
> 
> I'm proposing this during rampdown so the release ships with the latest version, as we also do with e.g. tzdata.

Marked as reviewed by serb (Reviewer).

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PR Review: https://git.openjdk.org/jdk8u/pull/83#pullrequestreview-3633131099