Change "Toolbox" references to "Toolbx"

[strugee]

See https://debarshiray.wordpress.com/2021/11/10/toolbox-is-now-toolbx/

Language here attempts to mirror CoreOS docs:

• "A toolbox" is unchanged
• References to the toolbox binary are unchanged (or clarified)
• "Toolbox container" is changed to "Toolbx container"
• The name of the project itself is changed

[strugee]

Note that I didn't have time to grep around for mentions in other pages.