Remove EnglishStemAnalyzer

[LoayGhreeb]

An issue occurs when using a custom analyzer I don't know why it behaves differently from the built-in analyzers in Lucene, even though the implementation is the same.

The issue occurs when searching with uppercase letters with wildcard characters. The custom analyzer does not ignore the case sensitivity as expected.

To reproduce the issue:

1. Use search in files.
2. Search using a query like "Kopp*".
3. Search using a query like "kopp*".
4. Both searches should yield the same results, but they do not.

Instead of using a custom analyzer, I used Lucene's EnglishAnalyzer, which is very similar to the custom analyzer we used. The main difference is that the custom analyzer uses DecimalDigitFilter.

I tried to use the exact same filters that the EnglishAnalyzer uses, but the same issue persists.

Lucene EnglishAnalyzer implementation:
https://github.com/apache/lucene/blob/2c81649e284b1a1f3a4b46fd589befc87306d0dc/lucene/analysis/common/src/java/org/apache/lucene/analysis/en/EnglishAnalyzer.java#L102-L110

Mandatory checks

• 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 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.