deprecated GoogleGroupChecker constructor seems broken

I was encountering NullPointer exceptions when updating the `play-googleauth` version in our project.

I might be doing something wrong but it seems to me like the deprecated GoogleGroupChecker constructor that takes a `GoogleServiceAccount` doesn't build an equivalent `ServiceAccountCredentials` properly.

It sets **privateKey** and **serviceAccountUser**`:
https://github.com/guardian/play-googleauth/blob/d60875f5edf1ad8fa4323a659a35ba02317a4305/play-v27/src/main/scala/com/gu/googleauth/groups.scala#L61-L69

But the `GoogleServiceAccount` class requires **clientEmail** to be set (see code [here](https://github.com/googleapis/google-auth-library-java/blob/v0.22.0/oauth2_http/java/com/google/auth/oauth2/ServiceAccountCredentials.java#L138) )

It might be that we just should be setting **clientEmail** instead of, or in addition to **serviceAccountUser** I'm not sure what the difference is exactly.
I just refactored my code to not use this method anyway so It's not blocking anything on my side.

Start in play-v27/src/main/scala/com/gu/googleauth/groups.scala at the deprecated GoogleGroupChecker constructor around lines 61–69, then compare its credential construction with ServiceAccountCredentials.java. Reproduce the reported NullPointerException and verify that the deprecated constructor creates equivalent credentials without the failure.

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