Fetch libxml2 from original source

What does this MR do?

Seems that under the current network situation we can get the libxml2 directly from original site, thus a mirror cos is not required.

Also fixes the build failure introduced by updating libxml2 version.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

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• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
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Expected (please provide an explanation if not completing)

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