[jfx21u] RFR: 8331748: Update libxml2 to 2.12.6

[Johan Vos]

8331748: Update libxml2 to 2.12.6

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Commit messages:
 - Backport 97b1402501983f121f75c24a510f466837fa2ecc

Changes: https://git.openjdk.org/jfx21u/pull/58/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx21u&pr=58&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331748
  Stats: 24912 lines in 115 files changed: 8485 ins; 11207 del; 5220 mod
  Patch: https://git.openjdk.org/jfx21u/pull/58.diff
  Fetch: git fetch https://git.openjdk.org/jfx21u.git pull/58/head:pull/58

PR: https://git.openjdk.org/jfx21u/pull/58