Remove POP3 support

Remove support for the POP3 protocol.

K-9 Mail never supported POP3's "download then delete from server" model. The app tries to treat POP3 accounts similar to IMAP where we download copies of messages but treat the server as single source of truth. Some settings exist to weaken that model, but the goal of #4970 is to get rid of these, too.

The hardest part about the removal is a deprecation strategy. We'll have to inform users that support for POP3 is going away before we actually remove it. They should have plenty of time to prepare moving to another protocol or another email client. I propose we show a non-dismissable info message above the message list. Clicking it could lead to a blog post with more information on the topic.

I don't want us to automatically delete user data. So once POP3 support is removed I propose we convert remaining POP3 accounts to read-only accounts. Users will still be able to view messages. They won't be able to copy, move, delete, or compose new messages from that account.

This is part of the effort to remove support for local messages.

Start by tracing the POP3 account flow and the message-list entry point described in the issue, then review the linked effort in #4970. Map the deprecation notice and the conversion of remaining POP3 accounts to read-only behavior; done means POP3 support is removed without automatically deleting user data.