Documentation: Update Debian dependency list

Apparently, qemu-system-* changed its name.

On Debian/Ubuntu, genext2fs does not make a difference: The only user of
this command, Meta/build-image-qemu.sh, runs as root anyway, and tries
to do 'mount _disk_image mnt/' first. Debian/Ubuntu always have drivers
for ext2 available, so this command has no good reason to fail, and
trying to use genext2fs might only obscure any problems that mount
encountered.

Finally, Debian Bullseye (released 2021-08-14) contains gcc-10, so the
instructions on how to upgrade are now obsolete.

Documentation/BuildInstructions.md

@@ -7,7 +7,7 @@ Make sure you have all the dependencies installed:
 ### Debian / Ubuntu
 
 ```console
-sudo apt install build-essential cmake curl libmpfr-dev libmpc-dev libgmp-dev e2fsprogs ninja-build qemu-system-i386 qemu-utils ccache rsync genext2fs unzip texinfo
+sudo apt install build-essential cmake curl libmpfr-dev libmpc-dev libgmp-dev e2fsprogs ninja-build qemu-system-gui qemu-system-x86 qemu-utils ccache rsync unzip texinfo
 ```
 
 #### GCC 10
@@ -18,8 +18,6 @@ On Ubuntu gcc-10 is available in the repositories of 20.04 (Focal) and later - a
 sudo add-apt-repository ppa:ubuntu-toolchain-r/test
 ```
 
-On Debian your system must be on the _testing_ or _unstable_ branch as gcc 10 is not available on _stable_ or in the backports. If you want to switch from _stable_ to _testing_, see the instructions on the Debian website on [switching to testing](https://wiki.debian.org/DebianTesting). Alternatively, if you want to stay on _stable_, you can build SerenityOS in a Debian testing or Ubuntu [Docker](https://www.docker.com/) container.
-
 Now on Ubuntu or Debian you can install gcc-10 with apt like this:
 
 ```console