docs: Cleanup the not used variants for the UTF-8 migration

[bernhardkaindl]

docs: Cleanup the not used variants for the UTF-8 migration

Base commit to pass CI:

• Remove the temporary exclusion of xcp/dmv.py from the diff/patch-coverage analysis
• Update CI to pass again, fails currently due to a 0.1% calculation change.

A simple PR without product code changes:

• Fix CI
• Cleanup obsolete docs on possibilities to handle UTF-8 that we didn't use.

[Copilot]

Pull Request Overview

This PR cleans up documentation and CI configuration following UTF-8 migration work. The changes focus on removing obsolete documentation about unused UTF-8 handling variants and adjusting CI thresholds to fix failing tests.

• Remove temporary exclusion of xcp/dmv.py from diff coverage analysis
• Lower test coverage threshold from 96% to 95% to accommodate calculation changes
• Cleanup obsolete documentation sections about unused UTF-8 handling approaches

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tox.ini	Removes temporary exclusion from diff coverage and adjusts test coverage threshold
README-Unicode.md	Removes documentation for unused UTF-8 handling variants, keeping only the implemented approach

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