Better PROXY_PROTOCOL

[nextgens]

What type of PR?

Feature

What does this PR do?

• Disable IMAP, POP3 and Submission by default; see https://nostarttls.secvuln.info/ on why explicit TLS is going away.
• Change the semantic of PROXY_PROTOCOL to make it configurable per port
• fix TLS_FLAVOR=notls not working with snappymail
• fix TLS_PERMISSIVE
• remove KUBERNETES_INGRESS; shouldn't be needed anymore
• update the documentation and the reverse proxy example

Related issue(s)

• close snappymail integration broken with TLS_FLAVOR=notls in master #3162
• close Allow PROXY_PROTOCOL to be per-port customizable. #3061

Prerequisites

Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

• In case of feature or enhancement: documentation updated accordingly
• Unless it's docs or a minor change: add changelog entry file.

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[Diman0]

It looks good to me. My current setup with proxy_protocol still works fine with this PR.
There are only two typo's that must be addressed. See in-line comments.

Pull request has been modified.

[nextgens]

Great catch; not sure how I have managed to typo that.

[Diman0]

LGTM

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