Replace System.String with ReadOnlySpan<System.Char> in Utf8YamlEmitter.WriteString

[ArchLeaders]

Changes the input parameter of Utf8YamlEmitter.WriteString from System.String to System.ReadOnlySpan<char> for more optimal writing scenarios.

Note, this PR changes the project LangVersion from 9 to 11 to use pattern matching on a ReadOnlySpan<char>.