Can't use SFTP for Backup Migrate in Drupal 9 due to package madness

Problem/Motivation

I can't get SFTP destinations working in Drupal 9 for reasons:

1. 'drupal/flysystem_sftp' is abandoned. It depends on Drupal 8.
2. I've submitted a MR to move this code to 'drupal/flysystem': #3354667: Add SFTP support to core module. Makes sense to include it, there is already FTP support.
3. SFTP support in 'league/flysystem' is now provided by a different package: 'league/flysystem-sftp-v3' which requires league/flysystem 3.
4. I can't require that library because it breaks drupal/backup_migrate_flysystem because it requires 2.x

This means I can't add SFTP support at all right now on anything higher than Drupal 8!

Steps to reproduce

On a Drupal 9 codebase:

composer require drupal/backup_migrate_flysystem drupal/flysystem_sftp

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - drupal/flysystem_sftp[dev-1.x, 1.x-dev] require drupal/core ~8.0 -> found drupal/core[8.0.0-beta6, ..., 8.9.x-dev] but the package is fixed to 9.5.3 (lock file version) by a partial update and that version does not match. Make sure you list it as an argument for the update command.

Proposed resolution

Add 3.x branch of flysystem as a possible dependency.

Remaining tasks

A release would be nice.

User interface changes

None

API changes

None.

There are no significant changes in the 3.x branch: https://git.drupalcode.org/project/flysystem/-/compare/2.0.x...3.0.x?fro...

Data model changes

None