Deprecate Domain expires_on in favor of expires_at

[duduribeiro]

We deprecated expires_on attribute in our documentation and api in
favor of expires_at. This commit introduces the new field and removes
the old one. It also updates to use the new fixtures.

We should do a major bump, since we are removing an attribute.

Also synced these new fixtures with the docs one

[OleMchls]

This would result in a breaking change and thus a new major release correct? Just want us to take that decision deliberately.

[jacegu]

@duduribeiro the only thing to keep in mind before releasing this is that clients should remain as much in sync as possible. So, if we end up not removing expires_on in the Ruby client, we should do the same in this one.

[duduribeiro]

@jacegu and @OleMchls since I made change, wanted a new 👁️ on it.

@OleMchls with this new change, we will follow the same as we are doing in the other libraries, to not make a hard move and don't have a major bump.

[weppos]

@duduribeiro the behavior in all the clients must be consistent. So this PR (and all the other clients) needs to be updated to behave as dnsimple/dnsimple-ruby#186

[duduribeiro]

@weppos @jacegu @OleMchls
Recently we dropped the domain expires_on on dnsimple-go by suggestion of @weppos, because dnsimple-go is not yet in 1.x, it wouldn’t be a problem to make another major release.
now on dnsimple-elixir, it is already at 2.0.0, we made a recent major release, so I made a little bit different from the dnsimple-go #135
I kept the expires_on, added a comment telling it is deprecated and added the expires_at. do you agree with it?