EVS - GPSSD and ESSD are not available with S6 family of flavors

[Bulat-Gumerov]

Examples shows that we can use SAS, SSD, GPSSD and ESSD, but in fact we can only select either SAS or SSD. GPSSD and ESSD are not available to use with Terraform.

terraform-provider-g42cloud/docs/resources/compute_instance.md

Line 227 in 46a4b5d

+ `GPSSD`: general purpose SSD disk type.

terraform-provider-g42cloud/docs/resources/evs_volume.md

Line 56 in 46a4b5d

+ SAS: specifies the high I/O disk type.