"auth-proxy-set-headers" should accept a configmap name without namespace

[isihu]

What happened:
When specifying a configmap for extra headers to send to the auth-proxy living in the same namespace I couldn't reach my service anymore.

# works
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: hello-world
  namespace: foo
  annotations:
    nginx.ingress.kubernetes.io/auth-url: http://auth.default.svc.cluster.local:8080/get
    nginx.ingress.kubernetes.io/auth-proxy-set-headers: foo/bar

# doesn't work
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: hello-world
  namespace: foo
  annotations:
    nginx.ingress.kubernetes.io/auth-url: http://auth.default.svc.cluster.local:8080/get
    nginx.ingress.kubernetes.io/auth-proxy-set-headers: bar

What you expected to happen:
Nginx should be able to read the name of the configmap living in the same namespace as stated in the docs.

NGINX Ingress controller version

-------------------------------------------------------------------------------
NGINX Ingress controller
  Release:       v1.9.4
  Build:         846d251814a09d8a5d8d28e2e604bfc7749bcb49
  Repository:    https://github.com/kubernetes/ingress-nginx
  nginx version: nginx/1.21.6

-------------------------------------------------------------------------------

PR that fixes this issue
#12997

[k8s-ci-robot]

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