instructions for installing on ubuntu use `apt-key`, which is deprecated

[ofloveandhate]

On the main Readme for this repo, for Ubuntu I am offered instructions that include running line

curl http://robotpkg.openrobots.org/packages/debian/robotpkg.key | sudo apt-key add -

but on Ubuntu 25, this fails because apt-key is deprecated.

[nim65s]

Thanks for the heads-up :)
This is fixed in #553