update changelog for v0.12.0

Merge pull request #176 from paulmach/pm/generic-geojson

geojson: add generic property support to features and collections

Merge pull request #177 from paulmach/cleanup

reduce go.mod's goversion 1.25 -> 1.18 for better compatibility

interface{} -> any

Merge pull request #174 from lukasbindreiter/master

Update bson encoding library to mongo-driver/v2

Merge pull request #172 from OlafFlebbeBosch/master

upgrade golangci-lint and fix errors

upgrade go.mod's goversion 1.15 -> 1.25

fix: adding subdirs so it can be found from the docs

ci(go): utilize go.mod for go version (#159)

update changelog for v0.12.0

geojson: update the rest of the CustomJSONMarshaler comments