ethminer - No OpenCL platforms found for AMD Radeon HD 3870 x2

I am trying to mine ethereum with an ancient laptop just for fun, but I can't get it running. I get this error:

ethminer 0.19.0-alpha.0
Build: linux/release/gnu
No OpenCL platforms found
Error: No usable mining devices found

Here's the setup:

GPU: AMD Radeon HD 3870 x2
OS: ubuntu server 20.04
Miner: ethminer 0.19.0-alpha-0

Running this command...

sudo lshw -c video | grep driver

gives this:

configuration: driver=radeon latency=0

What might be the problem? Is the 20.04 kernel incompatible with the drivers? Is everything too old?

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