products-page-generator: remove dependency on github.com/imdario/mergo

[jdolitsky]

see comment here: #112 (comment)

[jdolitsky]

Looks like it is coming from sprig which is used for some of the template functions:

$ go mod why github.com/imdario/mergo
# github.com/imdario/mergo
github.com/opencontainers/oci-conformance/products-page-generator
github.com/Masterminds/sprig
github.com/imdario/mergo