Remove DSSKey references from SSH provider, bump paramiko to 4.0+

[joshuabvarghese]

closes: #54079

Summary

• Remove paramiko.DSSKey references from SSHHook (_pkey_loaders and _host_key_mappings)
• Bump paramiko minimum to >=4.0.0 in the ssh and sftp providers (previously capped at <4.0.0 in Limit paramiko to < 4.0.0 till we remove DSS support #54078 as a stopgap)
• Bump types-paramiko in devel-common/pyproject.toml to drop the matching <4.0.0 cap added in Restrict typing-paramiko to < 4 as we do with paramiko #54173
• Add a migration note to the SSH connection docs pointing DSA-key users to RSA/ECDSA/Ed25519
• Add a significant newsfragment documenting the breaking change

Motivation

paramiko 4.0 removed DSSKey (DSA) support entirely for security reasons. #54078 temporarily capped paramiko<4.0.0 to keep CI green, and #54173 did the same for types-paramiko, both explicitly deferring the real fix to this issue.

This PR does the follow-through: drops the DSA loader/host-key-mapping code paths so the provider no longer references a class that doesn't exist in modern paramiko, and lifts both version caps.

Migration

Users with SSH/SFTP connections configured to use a DSA (DSS) private key will need to generate a new RSA, ECDSA, or Ed25519 key (e.g. ssh-keygen -t ed25519) and update the connection's key_file/private_key accordingly. This is called out in the newsfragment and in the connection docs.

Testing

• pytest providers/ssh/tests/unit/ssh/hooks/test_ssh.py
• pytest providers/sftp/tests/unit/sftp/hooks/test_sftp.py
• Verified paramiko.DSSKey no longer exists under paramiko>=4.0.0 (confirms the prior code path would have raised AttributeError at import time, matching the CI failure in Limit paramiko to < 4.0.0 till we remove DSS support #54078)

---

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[github-actions]

uv.lock on main just moved via #69198 ("[main] Upgrade important CI environment"), commit 38b2869 and this PR currently conflicts.

Quickest fix:

git fetch upstream main && git rebase upstream/main
rm uv.lock && uv lock
git add uv.lock && git rebase --continue
git push --force-with-lease

Automated nudge — ignore if you're not ready to rebase. This comment is updated in place on future uv.lock bumps.

[nailo2c]

Thanks for contributing! Could you help fix the CI error? I also found a regression issue that might need to be fixed :)

[nailo2c]

I think we should add a handler or document to avoid regression. For example, if users configured ssh-dss in host_key and then upgrade the ssh provider, this can fail with a KeyError.

e.g.
A connection like this:

Connection(
  conn_id="ssh_with_dss_host_key",
  host="localhost",
  conn_type="ssh",
  extra=json.dumps({"host_key": "ssh-dss AAAAB3NzaC1kc3MAAAAA"}),
)

would fail here because dss has been removed from _host_key_mappings:

airflow/providers/ssh/src/airflow/providers/ssh/hooks/ssh.py

Lines 229 to 232 in 4d56e6c

	if host_key is not None:
	if host_key.startswith("ssh-"):
	key_type, host_key = host_key.split(None)[:2]
	key_constructor = self._host_key_mappings[key_type[4:]]

[nailo2c]

Also, add a small regression test might be better I think :)