ssh/knownhosts: fix bracket normalisation

[pacien]

1. When using the bracketed syntax, the port is mandatory, even if
   it's the default one. Otherwise, OpenSSH rejects it with:
   "address [abcd:abcd:abcd:abcd]: missing port in address".
   See sshd(8): SSH_KNOWN_HOSTS FILE FORMAT.

2. Brackets are not necessary when using the default port,
   even for IPv6 addresses.

Fixes golang/go#53463

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Message from Gopher Robot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
A maintainer will review your change and provide feedback. See
https://go.dev/doc/contribute#review for more info and tips to get your
patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11 or adds a tag like "wait-release", it means that this CL will be
reviewed as part of the next development cycle. See https://go.dev/s/release
for more details.

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