Remove old warning about istio and external-domain-tls

[ReToCode]

Changes

• Remove old warning about istio and external-domain-tls
• Ref: https://cloud-native.slack.com/archives/C04LMU0AX60/p1718290650661339

[ReToCode]

/assign @dprotaso

/cherry-pick release-1.14

[knative-prow-robot]

@ReToCode: once the present PR merges, I will cherry-pick it on top of release-1.14 in a new PR and assign it to you.

In response to this:

/assign @dprotaso

/cherry-pick release-1.14

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[dprotaso]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, ReToCode

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• docs/serving/OWNERS [ReToCode,dprotaso]

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[knative-prow-robot]

@ReToCode: new pull request created: #6018

In response to this:

/assign @dprotaso

/cherry-pick release-1.14

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.