K8s v1.21 causing Valut integration outages (with kubernetes auth method)

Issue

On Kubernetes 1.21, the ServiceAccount issuer Discovery feature is on stable release and is enabled by default.

This means that the JWT format of the service accounts is changing to have a more secure format.

Previous format:

{
  "iss": "kubernetes/serviceaccount",
  "kubernetes.io/serviceaccount/namespace": "spinnaker",
  "kubernetes.io/serviceaccount/secret.name": "test-token-5v2cp",
  "kubernetes.io/serviceaccount/service-account.name": "test",
  "kubernetes.io/serviceaccount/service-account.uid": "0ecb5560-7d43-4883-ae85-d07cf635d2d2",
  "sub": "system:serviceaccount:spinnaker:test"
}

New format:

{
  "aud": [
    "https://kubernetes.default.svc"
  ],
  "exp": 1661509326,
  "iat": 1629973326,
  "iss": "https://oidc.server.something",
  "kubernetes.io": {
    "namespace": "spinnaker",
    "pod": {
      "name": "debugging-tools-6464df994b-46wsq",
      "uid": "90451169-29cb-4e2d-8ee8-4c1e2c293a3c"
    },
    "serviceaccount": {
      "name": "test",
      "uid": "affc78ef-fa4b-4ba8-bb00-f9cc51d65408"
    },
    "warnafter": 1629976933
  },
  "nbf": 1629973326,
  "sub": "system:serviceaccount:spinnaker:test"
} 

Cause

This breaks the vault kubernetes auth method with vault throwing the message:***ISS claim invalid***

This is causing Spinnaker and Spinnaker-operator not to be able to retrieve secrets from Vault***``````***