Use defusedxml as more secured xml parser (bsc#1227577)

[meaksh]

to be merged after 5.1 GM

#10398 (comment)

What does this PR change?

This PR makes use of defusedxml in rhnlib and spacewalk-backend-server to ensure more secure xml parsing.

NOTE: Since this PR is introducing new dependencies for rhnlib and spacewalk-backend-server, we need to coordinate with RelEng to provide the missing dependencies to the respective channels.

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: only internal and user invisible changes

• DONE

Test coverage

• No tests: already covered

• DONE

Links

Issue(s): https://github.com/SUSE/spacewalk/issues/24784

• DONE

Changelogs

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• Re-run test "backend_unittests_pgsql"
• Re-run test "java_pgsql_tests"
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• Re-run test "javascript_lint"
• Re-run test "spacecmd_unittests"

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[vzhestkov]

LGTM 👍

[meaksh]

Agreed with @rjmateus to postpone this after 5.1 GMC.

[deneb-alpha]

Agreed with @rjmateus to postpone this after 5.1 GMC.

I think we should also drop the merge-candidate label right? I fear that the PR could be merged before by mistake.