Deprecate the users ssh-authorized-keys property (#5162)

Jira: https://issues.redhat.com/browse/RHEL-45263

Backport the following fixes:

(a) https://github.com/canonical/cloud-init/commit/5205b4dd74eb2168ebbeba56579b6f116a272937

(b) https://github.com/canonical/cloud-init/commit/7c67f7732f04b41600934818f7d5bcb4d085ed7c

(c) https://github.com/canonical/cloud-init/commit/b3618d44a37ae6345f0c3d935b77ae0ae9dd1c92

Conflicts when they occurred and the reasons are mentioned in the commits itself.

tox is green in my environmment:

--------------------------------------------------------------------
Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)

  py3: OK (87.10=setup[8.32]+cmd[78.78] seconds)
  black: OK (2.48=setup[2.07]+cmd[0.41] seconds)
  ruff: OK (2.09=setup[2.03]+cmd[0.05] seconds)
  isort: OK (2.89=setup[2.04]+cmd[0.84] seconds)
  mypy: OK (3.91=setup[2.06]+cmd[1.85] seconds)
  pylint: OK (15.97=setup[2.27]+cmd[13.70] seconds)
  congratulations :) (114.49 seconds)