mapToUser/mapToGrantedAuthority in JdbcUserDetailsManager to become protected

[flozano]

Expected Behavior

I'd like to be able to override mapToUser and mapToGrantedAuthority in order to be able to customize how the user is mapped and what's the actual class to be used, and what's the instance of GrantedAuthority to be used.

Current Behavior

	private UserDetails mapToUser(ResultSet rs, int rowNum) throws SQLException {

Context
JdbcUserDetailsManager is not final and can be extended to override some of its behavior. It already has protected methods. By making this one protected, the extensibility can be much simplified.

The loadUsersByUsername is protected and can be extended already, so this is a minimal change.

[jzheaux]

Closing in favor of #16561