[hotfix-1.69] Remove kubernetes.io/ingress.class annotation in values.yaml

[petersutter]

What this PR does / why we need it:
Fixes an issue where the helm deployment failed with the error

Helm upgrade failed: failed to create resource: Ingress.extensions "gardener-dashboard-ingress" is invalid: annotations.kubernetes.io/ingress.class: Invalid value: "nginx": can not be set when the class field is also set

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

Fixed an issue where the helm deployment failed with the error `annotations.kubernetes.io/ingress.class: Invalid value: "nginx": can not be set when the class field is also set`

The default ingress class annotation under `Values.global.dashboard.ingress.annotations['kubernetes.io/ingress.class']` will not be set anymore. Instead, the ingress class name will be set using `Values.global.dashboard.ingress.ingressClassName`

[petersutter]

cc @JensAc - thanks for reporting

[holgerkoser]

/lgtm