AK: Use an enum instead of a bool for String::replace(all_occurences)

This commit has no behavior changes.

In particular, this does not fix any of the wrong uses of the previous
default parameter (which used to be 'false', meaning "only replace the
first occurence in the string"). It simply replaces the default uses by
String::replace(..., ReplaceMode::FirstOnly), leaving them incorrect.

Userland/Libraries/LibUnicode/DateTimeFormat.cpp

@@ -246,7 +246,7 @@ static Optional<String> format_time_zone_offset(StringView locale, CalendarPatte
 
     // The digits used for hours, minutes and seconds fields in this format are the locale's default decimal digits.
     auto result = replace_digits_for_number_system(*number_system, builder.build());
-    return formats->gmt_format.replace("{0}"sv, result);
+    return formats->gmt_format.replace("{0}"sv, result, ReplaceMode::FirstOnly);
 }
 
 // https://unicode.org/reports/tr35/tr35-dates.html#Time_Zone_Format_Terminology