Rename protoc-gen-swagger to protoc-gen-openapiv2 (#26243)

This rename happened *checks notes* [5.5 years ago](https://github.com/grpc-ecosystem/grpc-gateway/releases/tag/v2.0.0), whoops!

Closes #26243

COPYBARA_INTEGRATE_REVIEW=#26243 from johanbrandhorst:patch-2 04c0b96
PiperOrigin-RevId: 885319244

Author: johanbrandhorst

docs/options.md

@@ -196,7 +196,7 @@ about your project (name and website) so we can add an entry for you.
     *   Website: https://github.com/MarquisIO/go-grpcmw
     *   Extensions: 1041
 
-1.  grpc-gateway protoc-gen-swagger
+1.  grpc-gateway protoc-gen-openapiv2
 
     *   Website: https://github.com/grpc-ecosystem/grpc-gateway
     *   Extensions: 1042