Update the Raspberry Pi requirements in the install page

Why is this change being made?

In the install page we mention that 4 GB of RAM is recommended to run GitLab, but the Rpi info was about the Rpi2. We know have Rpi 4 with 4GB of RAM and this should be the recommended one. The Raspberry Pi folks also announced the renaming of Raspbian to Raspberry Pi OS, which is perfect, as we get to mention a distribution instead of a platform. We only did that with the Pi.

Since we now support Debian Buster, mention that as well.

I also opened an MR in the Omnibus project to rectify this gitlab-org/omnibus-gitlab!4081 (merged).

Relates to gitlab-org/gitlab#211814 (closed)

Closes #4742 (closed)