[main-lts] Fix #852 - Process Qute templates in a strict manner

[github-actions]

Backport: #864

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In this PR:

• Removed XML support on template code generation. It was never tested, possibly unstable.
• Turn Qute Strict processing, see more: https://quarkus.io/guides/qute-reference#strict_rendering
• Completely review of *.qute templates, variables, processing

I'm surprised a few of these features were working - at all—with many parsing logic and errors.

Fix #852

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[github-actions]

🎊 PR Preview e230b96 has been successfully built and deployed. See the documentation preview: https://quarkus-openapi-generator-preview-pr-870.surge.sh