ssh-add prints non-errors to stderr

[Bilin Sun]

I recently updated to 24.04, and switched from the default Wayland environment to the X11 environment (by selecting "Ubuntu on Xorg" instead of "Ubuntu" on the login screen) for better compatibility with some software. After the switch, every time I logged in, I got a dialog window saying

> Error found when loading /home/user/.profile:
> Identity added: /home/user/.ssh/id_ed25519
> As a result the session will not be configured correctly. You should fix the problem as soon as feasible.

This turned out to be caused by the line `ssh-add ~/.ssh/id_25519` which I put in my `.profile` earlier. Once I changed it to `ssh-add ~/.ssh/id_25519 2>/dev/null`, this dialog stopped showing up again.

I believe that "Identity added" should not be considered an error, and should be printed to stdout rather than stderr.