Tidy up CHANGELOG in preparation for release candidate (agda#2165)

Agda standard library version 1.7.3

Agda standard library version 1.7.2

Agda standard library version 1.7.1

Agda standard library version 1.7

Agda standard library version 1.6

Agda standard library version 1.5

Agda standard library version 1.4

Agda standard library version 1.3

Agda standard library version 1.2