ci: Filter supported versions of GKE

[marseel]

Whenever GKE stopped supporting a particular version of GKE, we had to
manually remove it from all stable branches. Now instead of that, we
will dynamically check if it's supported and only then run the test.

I will follow-up with the same approach for other cloud-providers.

[marseel]

/ci-gke

[marseel]

Example run where I added 1.23 version to matrix:
https://github.com/cilium/cilium/actions/runs/8922144762/job/24503735567

Fetching server config for us-west2-c
Removing version 1.23 as it's not valid for zone us-west2-c
Fetching server config for us-west2-c
Version 1.26 is valid for zone us-west2-c
Fetching server config for us-west3-a
Version 1.27 is valid for zone us-west3-a
Fetching server config for us-east4-b
Version 1.28 is valid for zone us-east4-b
Fetching server config for us-east1-c
Version 1.29 is valid for zone us-east1-c
Filtered matrix:
{
  "k8s": [
    {
      "version": "1.26",
      "zone": "us-west2-c",
      "vmIndex": 1
    },
    {
      "version": "1.27",
      "zone": "us-west3-a",
      "vmIndex": 2
    },
    {
      "version": "1.28",
      "zone": "us-east4-b",
      "vmIndex": 3
    },
    {
      "version": "1.29",
      "zone": "us-east1-c",
      "vmIndex": 4,
      "default": true
    }
  ],
  "config": [
    {
      "type": "no-tunnel",
      "index": 1
    },
    {
      "type": "tunnel",
      "index": 2,
      "cilium-install-opts": "--datapath-mode=tunnel"
    },
    {
      "type": "ipsec",
      "index": 3,
      "cilium-install-opts": "--helm-set=encryption.enabled=true --helm-set=encryption.type=ipsec"
    },
    {
      "type": "tunnel-ipsec",
      "index": 4,
      "cilium-install-opts": "--helm-set=encryption.enabled=true --helm-set=encryption.type=ipsec --datapath-mode=tunnel"
    }
  ]
}

[marseel]

/test

[marseel]

/ci-gke

[marseel]

/test

[tklauser]

LGTM, only nits inline which can be addressed as a follow-up.