RFR: 8368691: Update libxml2 to 2.14.6

[Hima Bindu Meda]

Updated libxml2 to v2.14.6. Verified build on all platforms. No issue seen.

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Commit messages:
 - version update
 - whitespace correction
 - configure libxml on windows
 - configure libxml on linux
 - update libxml to v2.14.6

Changes: https://git.openjdk.org/jfx/pull/1920/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1920&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368691
  Stats: 123 lines in 16 files changed: 74 ins; 11 del; 38 mod
  Patch: https://git.openjdk.org/jfx/pull/1920.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1920/head:pull/1920

PR: https://git.openjdk.org/jfx/pull/1920