Update upgrade md for new unified config for sql string mapping to utf8view when we release datafusion 49.0.0

[zhuqi-lucas]

Is your feature request related to a problem or challenge?

https://github.com/apache/datafusion/pull/16290 Has been merged, so we need to update the doc for next release.

New unified config for sql string mapping:

map_string_types_to_utf8view

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response

[alamb]

Added to list on https://github.com/apache/datafusion/issues/16235