[jfx18] RFR: 8286256: Update libxml2 to 2.9.14

[Johan Vos]

Clean backport of JDK-8286256

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Commit messages:
 - 8286256: Update libxml2 to 2.9.14

Changes: https://git.openjdk.org/jfx/pull/834/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=834&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8286256
  Stats: 21664 lines in 81 files changed: 4429 ins; 4294 del; 12941 mod
  Patch: https://git.openjdk.org/jfx/pull/834.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/834/head:pull/834

PR: https://git.openjdk.org/jfx/pull/834