Update Debian/Ubuntu section of installing-atom.md

[ThamesDev]

I noticed that a section of your installation guide for Linux used apt-key, which is a deprecated command. I opened a pull request to fix this, but I wondered whether I should also open an issue. I hope you'll excuse me if I haven't gone through the proper channels, as I'm not quite sure what they are - this is my first contribution and I'm very new to GitHub. Thanks in advance!

[Kodar-le-dok]

The fix from @ThamesDev is still pending, so I write here the steps I used to fix it on my computer (Debian 12):
at section "Debian and Ubuntu (deb/apt)", use the following lines in the first code bloc

wget -O- https://packagecloud.io/AtomEditor/atom/gpgkey | gpg --dearmor | sudo tee /usr/share/keyrings/atom-archive-keyring.gpg
sudo sh -c 'echo "deb [arch=amd64 signed-by=/usr/share/keyrings/atom-archive-keyring.gpg] https://packagecloud.io/AtomEditor/atom/any/ any main" > /etc/apt/sources.list.d/atom.list'
sudo apt-get update

See here for more info on the syntax and the reason behind this necessary fix.