0.17.3 is the most recent version with an ARM64 build available.
I don't run Lemmy on a Raspberry Pi, but I do run it on an ARM64 server platform. I couldn't upgrade to 0.17.4 and also not to 0.18.0.
I tried to build 0.18.0 myself today and kinda succeeded, but ran into some minor issues and rolled back for now, because it's too late and I'm tired.
I will write up some steps to get 0.18.0 running on Monday, as Sunday is my wife's birthday.