Remove gulpfile from the MVC UI projects

[hikalkan]

Since we now have the abp install-libs command. Also, check the documentation to offer the new usage instead of yarn && gulp.

[NecatiMeral]

This change requires our CI agents to have the ABP CLI installed. I personally feel like the more native way of yarn && gulp is more suitable for CI/CD scenarios.

Will the old behavior still be supported?

[hikalkan]

We won't maintain the old gulp file, but not remove it from the npm package. However, removing it from new startup templates beginning from v5.0 (end of this year).