[jdk11u-dev] RFR: Merge jdk11u:master

[Andrew John Hughes]

Merge jdk-11.0.30+6

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

Commit messages:
 - Merge jdk11u:master
 - 8372534: Update Libpng to 1.6.51
 - 8317970: Bump target macosx-x64 version to 11.00.00

The merge commit only contains trivial merges, so no merge-specific webrevs have been generated.

Changes: https://git.openjdk.org/jdk11u-dev/pull/3137/files
  Stats: 645 lines in 22 files changed: 297 ins; 210 del; 138 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/3137.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/3137/head:pull/3137

PR: https://git.openjdk.org/jdk11u-dev/pull/3137