Update MLIR's default SM to `sm_75`. This matches the behavior of
offline compilation tools in the CUDA Toolkit (`nvcc`, `ptxas`, ...) and
follows suit with 9fc5fd0ad689eed94f65b1d6d10f9c5642935e68.
Additionally, `sm_75` is the oldest GPU variant compatible with the
widest range of recent major CUDA Toolkit versions (11/12/13).
With the removal of mlir-vulkan-runner (as part of #73457) in
e7e3c45bc70904e24e2b3221ac8521e67eb84668, mlir-cpu-runner is now the
only runner for all CPU and GPU targets, and the "cpu" name has been
misleading for some time already. This commit renames it to mlir-runner.
