Update GORM example to use latest GO version + GORM v2 + pgx Smart Driver

[Sfurti-yb]

These changes are to:

• Update the app to use latest go functionalities
• Upgrade to gorm v2
• Use Yugabytedb go smart driver

Changes made:

1. README - Updated steps to run the app
2. Changed "github.com/jinzhu/gorm/dialects/postgres" => "github.com/yugabyte/gorm-yugabytedb" for yugabytedb smart driver
3. Changed "github.com/jinzhu/gorm" => "gorm.io/gorm" for GORM v2
4. Removed dbConn.Model(&model.Tablename{}).AddForeignKey() since it is no longer supported in GORM v2. Instead added the same constraints during model declaration
5. Removed explicit orderline table insert since it is done when order table insert is done.

Testing:
After running the app follow the steps here to do inserts: https://docs.yugabyte.com/stable/drivers-orms/orms/go/ysql-gorm/#send-requests-to-the-application
Note: For order creation, instead of :

curl \
  --data '{ "userId": "2", "products": [ { "productId": 1, "units": 2 } ] }' \
  -v -X POST -H 'Content-Type:application/json' http://localhost:8080/orders

use:

curl \
  --data '{ "userId": 2, "products": [ { "productId": 1, "units": 2 } ] }' \
  -v -X POST -H 'Content-Type:application/json' http://localhost:8080/orders

The docs have an error.

[HarshDaryani896]

LGTM, Please update gorm-yugabytedb to use the latest Yugabytedb pgx smart driver.

[HarshDaryani896]

nit: Remove Leave blank for the password.