Corne 5-column keymap should use physical-layout

[amacleod]

ZMK is moving towards preferring physical-layout over matrix-transform, and for builds that include ZMK Studio, physical-layout is required (see zmkfirmware/zmk@7276fd5).

In order to make it easier for people to add ZMK Studio to their firmware builds, and to set a good example, the Corne 5-column configuration should switch to using chosen { zmk,physical-layout = &foostan_corne_5col_layout; };