improve utf8 charset detection

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

Author: Siedlerchr

src/main/java/org/jabref/logic/importer/Importer.java

@@ -121,7 +121,8 @@ protected static Charset getCharset(BufferedInputStream bufferedInputStream) {
                 return defaultCharSet;
             }
 
-            if (Arrays.stream(matches).anyMatch(charset -> "ASCII".equals(charset.getName()))) {
+            // if we have utf8 with 100 confidence we
+            if (Arrays.stream(matches).anyMatch(charset -> "ASCII".equals(charset.getName()) || ("UTF-8".equals(charset.getName()) && charset.getConfidence() == 100))) {
                 return defaultCharSet;
             }