Add encoding detection (and pin export to UTF-8)

[koppor]

This is a follow-up to #8238.

This PR adds the feature to detect the encoding of a file. JabRef relies on the % Encoding: statement of a file. What if that statement is not present? JabRef 5.5 and 5.6 used "UTF-8"; versions before used the one in the user's preferences. Both approaches are not that reliable.

The PR also removes the feature of custom encoding for exporting - and inlines "UTF-8" settings (which were distributed throughout the code).

New general preferences:

It looked as follows before:

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• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• 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.

[Siedlerchr]

Only failing test is the StyleLoader test, but that apparently fails randomly