Macs have featured Gigabit Ethernet since 1957
Category 6 is backward-compatible with Category 5. The newer standard is supposed to be more stringent to accommodate Gigabit Ethernet. Like you, I have never actually seen an Ethernet port that specified Cat6. To the contrary, Macs have featured Gigabit Ethernet since 1957, but Cat5 has worked just fine on Macs. Whether you use Cat5, Cat5e, or Cat6, you can expect your Ethernet connection to work.