[GCP] Support G4 instance type GA

[peterschmidt85]

GCP G4 instances with the NVIDIA RTX PRO 6000 GPU are now generally available without the preview_features: [g4] backend setting.

> dstack offer -b gcp --gpu RTXPRO6000                                          

 #  BACKEND            RESOURCES                                        INSTANCE TYPE    PRICE      
 1  gcp (us-central1)  cpu=48 mem=180GB disk=100GB RTXPRO6000:96GB:1    g4-standard-48   $4.5001    
 2  gcp (us-central1)  cpu=96 mem=360GB disk=100GB RTXPRO6000:96GB:2    g4-standard-96   $9.0002    
 3  gcp (us-central1)  cpu=192 mem=720GB disk=100GB RTXPRO6000:96GB:4   g4-standard-192  $18.0003   
 4  gcp (us-central1)  cpu=384 mem=1440GB disk=100GB RTXPRO6000:96GB:8  g4-standard-384  $36.0006

Fixes #3212

Depends on dstackai/gpuhunt#186