Update string.rbs to avoid deprecation warning in Steep

[soutaro]

Both ::_ArefFromStringToString and String::encode_fallback are marked as deprecated, and referencing the interface from encode_fallback results in RBS::DeprecatedTypeName error in RBS file reported by Steep.

$ bundle exec steep check --no-type-check --validate=library
# Type checking files:

....................................................................................................................................................................................................F..............

core/string.rbs:3583:70: [warning] Type `::String::_ArefFromStringToString` is deprecated
│ Diagnostic ID: RBS::DeprecatedTypeName
│
└ type String::encode_fallback = Hash[String, String] | Proc | Method | String::_ArefFromStringToString
                                                                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Detected 1 problem from 1 file

This warning doesn't make sense:

1. String::encode_fallback itself is a deprecated, so using the type in user's code are already reported as a type error
2. Users cannot silence the warning by changing Steep configuration

This PR changes the type definition to remove the warning.