Use 127.0.0.1 instead of localhost as default in QOAuth2AuthorizationCodeFlow

In QTBUG-67528, it's established that 127.0.0.1 is preferred over localhost, it was however reverted back to localhost in 7d0995068ae67bad291b4e3066085700e7481758.

It's better to stick with 127.0.0.1, otherwise the urls in documentations should be updated.

• examples/oauth/redditclient/doc/src/qtnetworkauth-redditclient.qdoc
• examples/oauth/twittertimeline/doc/src/qtnetworkauth-twittertimeline.qdoc

 This is a breaking change, as the user might need to update their authorized redirect URL