Add OCI `experimental_instance_types` setting (dstackai#4329)

Add an OCI backend setting that accepts a list of
shape names to allow provisioning in addition to
the standard supported instance families. Only
works for shapes included in `dstack`'s pricing
catalog (`gpuhunt`).

This setting can be used to test various shapes
that were not yet tested internally at `dstack`.

```yaml
experimental_instance_types: [BM.GPU.RTXPRO.8]
```

Support ALB for GCP gateways (dstackai#4279)

`dstack` now creates and manages an Application
Load Balancer for GCP gateways when
`load_balancer` is set to `{ type: alb }`.

```yaml
type: gateway
name: gcp-gateway
backend: gcp
region: europe-west9
domain: example.com
load_balancer:
  type: alb
replicas: 2
certificate: null
public_ip: false
```

This allows you to have more than one GCP gateway
replica, with load balancing handled automatically
by `dstack`.

```shell
$ dstack gateway list
 NAME          BACKEND             HOSTNAME     DOMAIN       DEFAULT  STATUS
 gcp-gateway                       10.200.0.53  example.com  ✓        running
    replica=0  gcp (europe-west9)  10.200.0.54                        running
    replica=1  gcp (europe-west9)  10.200.0.55                        running
```

GCP ALB is supported for both `public_ip: false`
and `public_ip: true` gateways. Currently, it is
only supported with `certificate: null` (no
HTTPS).

Drop legacy `dstack-gateway` package (dstackai#4252)

Install the `dstack` package on gateway instances
directly. Introduce the
`DSTACK_GATEWAY_PACKAGE_URL` server environment
variable for overriding the installation source.

Support in-place update for service `gateway` (dstackai#4221)

The `gateway` property in service configurations
can now be updated in-place.

```shell
$ dstack apply -f my-service.dstack.yml

Active run my-service already exists. Detected changes that can be updated in-place:
- Configuration properties:
  - gateway

Update the run? [y/n]: y
 NAME        BACKEND          GPU  PRICE           STATUS   SUBMITTED
 my-service  aws (us-east-2)  -    $0.0005 (spot)  running  09:22

my-service provisioning completed (running)
Service is published at:
  http://my-service.second-gateway.example.com
```

This allows you to reassign a service to a
different gateway without stopping the service.

```shell
$ dstack event
[2026-08-27 09:28:33] [run my-service, gateway second-gateway] Service assigned to gateway
[2026-08-27 09:28:33] [👤admin] [run my-service] Run updated. Deployment: 1. Changed fields: configuration.gateway
[2026-08-27 09:28:39] [run my-service, gateway-replica first-gateway-0] Service unregistered
[2026-08-27 09:28:39] [job my-service-0-0, gateway-replica first-gateway-0] Service replica unregistered
[2026-08-27 09:28:41] [run my-service, gateway-replica second-gateway-0] Service registered
[2026-08-27 09:28:41] [job my-service-0-0, gateway-replica second-gateway-0] Service replica registered
```

Migrating between gateways would typically result
in service downtime while you are rerouting your
client traffic to the new gateway. During
reassignment, the service may also be unavailable
on both gateways for a brief period (several
seconds).

Migration from a gateway to running without a
gateway (`gateway: false`) and vice versa is also
supported.

Fix server compatibility with 0.20.[27-29] clients (dstackai#4148)

Ignore `GatewayConfiguration.router` in client
requests if `null`, which is what 0.20.27,
0.20.28, and 0.20.29 clients send by default.

Add 0.21 section to the upgrade guide (dstackai#4106)