Migrating to Protobuf V2 module

[himanshpal]

The https://github.com/golang/protobuf module has been superseded by the google.golang.org/protobuf module, which contains an updated and simplified API, support for protobuf reflection, and many other improvements. https://github.com/golang/protobuf is now marked as deprecated and gin still uses an older version of plugin

[ilovesusu]

golang/protobuf#1306

[thinkerou]

#2827 thanks