bugfix: use correct service-account-issuer in case of custom clusterDomain

[vardhaman-surana]

What issue type does this pull request address? (keep at least one, remove the others)
/kind bugfix

What does this pull request do? Which issues does it resolve? (use resolves #<issue_number> if possible)
resolves #2315

The PR adds the correct service account issuer to the apiserver in case custom clusterdomain is set using networking.advanced.clusterDomain.
It also fixes _coredns.tpl in the chart to correctly use custom clusterdomain in coredns configuration.

Please provide a short message that should be published in the vcluster release notes
Fixed an issue where vcluster was setting wrong service-account-issuer in case of custom clusterDomain.

What else do we need to know?

in case of the custom cluster domain the service account token for the vcluster pods is created with audiences

"https://kubernetes.default.svc.<custom-domain>", "https://kubernetes.default.svc", "https://kubernetes.default"

and because of the issuer set to https://kubernetes.default.svc.cluster.local the apiserver rejects the authentication request for the token. with the error

invalid bearer token, token audiences ["https://kubernetes.default.svc.<custom-domain>", "https://kubernetes.default.svc", "https://kubernetes.default"] is invalid for the target audiences ["https://kubernetes.default.svc.cluster.local"]

for k0s setting just the clusterDomain in the config file works. and for k3s users needs to use correct settings as below

networking:
  advanced:
    clusterDomain: "k8s.my-cluster"
controlPlane:
  distro:
    k3s:
      enabled: true
      extraArgs: ["--cluster-domain=k8s.my-cluster"]

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[FabianKramm]

@vardhaman-surana thanks for the PR! Looks good to me!

[loft-bot]

💔 Some backports could not be created

Status	Branch	Result
❌	v0.20	Backport failed because of merge conflicts
✅	v0.21
✅	v0.22

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