fix: Support both DSA and RSA public keys for OpenSSH 10.0+ compatibility

[Tarun-goswamii]

Summary

OpenSSH 10.0 (released 2025-04-09) removed DSA key support. This updates _load_pubkey() to recognize both DSA and RSA public keys, ensuring backward compatibility for existing users while supporting new installations.

Fixes the toolkit side of: sugarlabs/sugar#1004

Problem

The existing code only recognizes ssh-dss (DSA) prefix:

magic = 'ssh-dss '
for line in lines:
    line = line.strip()
    if not line.startswith(magic):
        continue
    return line[len(magic):]

#Solution
Support both key types:#

supported_key_types = ('ssh-dss ', 'ssh-rsa ')
for line in lines:
    line = line.strip()
    for magic in supported_key_types:
        if line.startswith(magic):
            return line[len(magic):]