Update the RDS provider documentation to specify that the username field is mandatory for defining the username of the master DB user.

[thiagoscodelerae]

The username field is not specified as required for DB instance creation on RDS provider documentation. The documentation should be updated to specify this field as required.

Also, using the default value "postgres" would be good to implement if we don't set any value. (This is what AWS does on the Console as the default username option).

Affected Resource(s) rds.aws.upbound.io/v1beta2 - Instance https://marketplace.upbound.io/providers/upbound/provider-aws-rds/v1.12.0/resources/rds.aws.upbound.io/Instance/v1beta2

Source of this request: https://github.com/crossplane-contrib/provider-upjet-aws/issues/1476

[mergenci]

Terraform documentation of the field states “Required unless a snapshot_identifier or replicate_source_db is provided”. Attributes of complex resources cannot be classified into a binary “required” or not. As suggested, Upjet should better include such caveats in the documentation.