Making coreutils umkdir as the default mkdir (#12007)

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# Description
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`umkdir` was added in #10785, I think it's time to replace the default
one.

# After Submitting

Remove the old `mkdir` command and making coreutils' `umkdir` as the
default

crates/nu-command/tests/commands/mod.rs

@@ -60,7 +60,6 @@ mod ls;
 mod match_;
 mod math;
 mod merge;
-mod mkdir;
 mod mktemp;
 mod move_;
 mod mut_;