refactor: XML validator explicitely harden against XXE injections

[jkowalleck]

Changed

• The provided XML validation capabilities are hardened (via #1064; concerns #1061)
  This is considered a security measure concerning XML external entity (XXE) injection.

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This is not an actual change.
Per default, the XML validation capabilities were already secure in the intended ways.
This is to prevent the fuckup like in the yanked v6.7.0 (see details here)

[codacy-production]

Coverage summary from Codacy

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Coverage variation	Diff coverage
Report missing for e7bc72e1	✅ 100.00% (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (e7bc72e)	Report Missing	Report Missing	Report Missing
Head commit (f5b9752)	22788	22377	98.20%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1064)	5	5	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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