Improve utf8 detection

[Siedlerchr]

Fixes https://discourse.jabref.org/t/jabref-5-15-opens-bib-files-with-shift-jis-encoding-instead-of-utf-8/5369/

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

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[HoussemNasri]

Suggested change

	- We fixed an issue where a bib file with UFF-8 charset was wrongly loaded with a different charset [forum#5369](https://discourse.jabref.org/t/jabref-5-15-opens-bib-files-with-shift-jis-encoding-instead-of-utf-8/5369/)
	- We fixed an issue where a bibtex file with UTF-8 charset was wrongly loaded with a different charset [forum#5369](https://discourse.jabref.org/t/jabref-5-15-opens-bib-files-with-shift-jis-encoding-instead-of-utf-8/5369/)