sftp: switch from libssh2 to libssh

libssh: https://www.libssh.org/

cf. vlc-android#1508 (comment 348022)

A solution could be moving to libssh (LGPL, no relation to libssh2) that supports these other host key algorithms without needing any apache code. Depends on what the devs of VLC think. But requiring users to enable ssh-rsa when it has been explicitly disabled in the latest mainstream distros (Ubuntu 22.04) is clearly not a great idea.

Edit: I refer to the time when qemu switched from libssh2 to libssh in 2018

TLDR; LGPL and more key algorithms

changed milestone to %Features paradize

added Component::Input: network streams label

changed the description

Potentially useful references:

• Table of libssh vs libssh2 by quendi.de
• libssh's license which is LGPL
• libssh usage examples, including SFTP documentation for sftp_async_read_begin,sftp_async_read, sftp_file_set_nonblocking, sftp_seek/sftp_seek64, sftp_attributes_free, sftp_close, sftp_free
• libssh's git
• libcurl maintains both a libssh backend and a libssh2 backend. Fedora has moved to use the libssh backend by default in libcurl in 2018.

libcurl currently uses libssh2 to implement the SSH layer of SCP and SFTP protocols. The libssh2 library uses outdated crypto algorithms and lacks important features like GSS-API authentication. After implementing this change, libcurl will use the libssh library instead, which is now more secure, feature-complete, and with more active upstream community.