Changed encoding used by IntegrityCheck

[k3KAW8Pnf7mkmdSMPHz27]

Fixes #8022.

Previous method used System.getProperty("file.encoding") to get the current character encoding).
Changed to use,

1. bibDatabaseContext.getMetaData().getEncoding()
2. preferencesService.getGeneralPreferences().getDefaultEncoding()

Todo:

1. I don't think it can distinguish GBK from UTF-8

• 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.

Refs issue #5850 and PR #7639

[k3KAW8Pnf7mkmdSMPHz27]

I should be able to test it in a running version of JabRef on Windows latest Monday.