Stuck in tty1 after updating

### What Happened?

The lightdm greeter does not launch when rebooting the OS after an update.

### Steps to Reproduce

1. Run `sudo apt update && sudo apt upgrade` in Terminal.
2. The OS notifies to restart after the update is finished.
3. Restart the OS.
4. After restart, lightdm does not launch and a tty appears with this text:
`elementary OS Jolnir elementary tty1`
`elementary login:`

### Expected Behavior

I expected lightdm to start when the OS starts to greet the user with a graphical login instead of a tty prompt.

### OS Version

6.x (Odin)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell
$ ps aux | grep lightdm
root 930 0.0 0.0 305852 7040 ? Ssl 17:22 0:00 /usr/sbin/lightdm
# my ps aux command showed up here as well.

$ startx
X.Org X Server 1.20.13
X Protocol Version 11, Revision 0
# attempts to start, but gives this message after crashing:
xinit: connection to X server lost

$ cat /var/log/Xorg.0.log | grep nvidia
# no output here
$ cat /var/log/Xorg.0.log | grep nouveau
[ 1451.666] (==) Matched nouveau as autoconfigured driver 0
[ 1451.666] (II) LoadModule: "nouveau"
[ 1451.669] (II) Loading /usr/lib/xorg/modules/drivers/nouveau_drv.so
[ 1451.671] (II) Module nouveau: vendor="X.Org Foundation"
```

### Hardware Info

AMD Athlon X4 860K
NVIDIA GTX 960 2GB
8GB DDR3 RAM
I installed `nvidia-driver-515` via apt and was using that prior to the update. `startx` also does not work as listed above in the Log Output. Based on the info from the `/var/log/Xorg.0.log` file, it appears to be attempting to use the nouveau driver now. I tried running `sudo ubuntu-drivers autoinstall`, but that did not install or change any packages.