CHANGELOG for 1.7.3 missing on master

[andreasabel]

Probably this directory should also contain a 1.7.3.md: https://github.com/agda/agda-stdlib/tree/dfd57bd3ff00e48fc5e8641e3303db624b6330f8/CHANGELOG

[jamesmckinna]

I think @MatthewDaggitt managed that release (indeed all the releases back to ..201x?), so pinging him...