migrate to StringUtil.toLower/UpperCase, ENGLISH locale

GitOrigin-RevId: 3a8627e7aef5991ee840996c6f7f147d1f747a3b

src/org/jetbrains/java/decompiler/main/Fernflower.java

@@ -32,7 +32,7 @@ public class Fernflower implements IDecompiledData {
     String level = (String)properties.get(IFernflowerPreferences.LOG_LEVEL);
     if (level != null) {
       try {
-        logger.setSeverity(IFernflowerLogger.Severity.valueOf(level.toUpperCase(Locale.US)));
+        logger.setSeverity(IFernflowerLogger.Severity.valueOf(level.toUpperCase(Locale.ENGLISH)));
       }
       catch (IllegalArgumentException ignore) { }
     }

src/org/jetbrains/java/decompiler/modules/renamer/ConverterHelper.java

@@ -33,7 +33,7 @@ public class ConverterHelper implements IIdentifierRenamer {
            !isValidIdentifier(elementType == Type.ELEMENT_METHOD, value) ||
            KEYWORDS.contains(value) ||
            elementType == Type.ELEMENT_CLASS && (
-             RESERVED_WINDOWS_NAMESPACE.contains(value.toLowerCase(Locale.US)) ||
+             RESERVED_WINDOWS_NAMESPACE.contains(value.toLowerCase(Locale.ENGLISH)) ||
              value.length() > 255 - ".class".length());
   }