Only scan for *.graphqls and *.gqls when looking for schema files

See #134 for background

Currently, the GraphQL auto-configuration scans for `*.graphql, *.gql, *.graphqls, *.gqls` files when looking for schema files during application startup. `*.graphql, *.gql` files seem to be generally used for queries instead.

We should reflect that in the default value - customization is still possible through changes done in #134.