`ads_insights_age_and_gender` stream `age` property with type "string or integer or null" causes some loaders to fail

In https://github.com/singer-io/tap-facebook/pull/10/files#diff-b56b9199b76c42734f7f511d063aa529991efee0e8af8eab8b002cc0b3d7b334L494-R477, the type of age was changed from "integer or null" to "integer or string or null", which causes issues in some loaders (most notably https://github.com/datamill-co/target-snowflake and https://github.com/datamill-co/target-postgres) because (in most databases?) a column can only have a single (nullable) type.

As age actually contains strings like "18-24" and "65+", not plain integers, we can change the type to "string or null".