Everywhere: Use ReadonlySpan<T> instead of Span<T const>

Author: https://github.com/MacDue
Commit: 63b11030f0
Pull-request: https://github.com/SerenityOS/serenity/pull/17328
Reviewed-by: https://github.com/trflynn89

Meta/Lagom/Tools/CodeGenerators/LibLocale/GenerateNumberFormatData.cpp

@@ -915,7 +915,7 @@ static Optional<NumberSystem> keyword_to_number_system(KeywordNumbers keyword)
     }
 }
 
-Optional<Span<u32 const>> get_digits_for_number_system(StringView system)
+Optional<ReadonlySpan<u32>> get_digits_for_number_system(StringView system)
 {
     auto number_system_keyword = keyword_nu_from_string(system);
     if (!number_system_keyword.has_value())