Canon (mirrorless) uses an RF mount. You can use your EF lens with the R6, but you'll need an adapter.
They make 2
The R6 is an awesome camera, the EF 75-300 lens is nowhere near that quality. I strongly recommend replacing it and getting either:
RF24-240 lens - no adapter needed, and OIS works with IBIS see a review with video HERE
RF 100-400 If you want to stick with at least the same zoom range: review and video HERE
If you prefer to stick the the EF mount, and get an adapter, then check out the EF 70-300 range (which you can likely pick up relatively cheaply) see my own reviews HERE