Update to released version of ember-cli-page-object v2

[ascudder]

Upgrades the version of ember-cli-page-object from the beta version (~v2.0.0-beta.3) to the stable released version (^2.1.0) so that it is compatible with a wider range of ember-cli-page-object versions. Additionally, updates
the engines specification to match the compatibility of ember-cli-page-object.

[mixonic]

Awesome. Yeah IIRC the reason this was pinned to the beta was because the dependency had dropped support for a version of Ember still supported by Ember Table (and used by Addepar at the time).

Happily that is no longer the case, and we will be cutting a new major of Ember Table Addepar/ember-table#995 with support for only Ember 3.28+ in the coming weeks.

Let me check if this seems like it is unblocked, and if so I could merge this and cut a new minor version (non beta) with this change.

[ascudder]

Thanks @mixonic for taking a look and for the explanation on compatibility. Fortunately it looks like we're already on a compatible version of Ember, so If this can make it into a pre-release version of the next release, that would help us out, too.