networkctl: shorten Minimum/Maximum to min/max

networkctl: shorten Minimum/Maximum to min/max

This makes the output slightly "lighter" visually. min/max are well-established
forms, so there's no risk of misunderstanding. Also, not using title-case looks
a bit better.

Follow-up for c06ff86e25db4fdb4e962baf812ed28d8d0c1050.

$ build/networkctl status '*'|grep MTU
                   MTU: 65536
                   MTU: 1500 (min: 68, max: 9000)
                   MTU: 1500 (max: 2048)
                   MTU: 1500 (min: 256, max: 2304)
                   MTU: 1500 (min: 68, max: 65535)
                   MTU: 1500 (min: 68, max: 65521)
                   MTU: 1500 (min: 68, max: 65535)
                   MTU: 1500 (max: 65535)
                   MTU: 1360 (min: 68, max: 65535)