ci: bump actions/checkout from 3 to 4 #86

Job	Run time
build (examples/a_wild_card/)	2m 36s
build (examples/b_address_change/)	2m 50s
build (examples/c_check_all_addresses/)	2m 28s
build (examples/d_simple_logger/)	2m 29s
build (examples/e_continuous_measurement/)	2m 36s
build (examples/f_basic_data_request/)	2m 38s
build (examples/g_terminal_window/)	2m 49s
build (examples/h_SDI-12_slave_implementation/)	2m 29s
build (examples/i_SDI-12_interface/)	1m 5s
build (examples/j_external_pcint_library/)	1m 17s
build (examples/k_concurrent_logger/)	1m 7s
	24m 24s

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