Use idiomatic return type for propertyMissing setter

[Vampire]

No description provided.

[Vampire]

With the current state compilation fails if you make a static nested class extend any of the integration classes that use propertyMissing like GebSpec , due to GROOVY-11822.

With this change released, it will at least compile properly even though the magic browser forwarding will not work, but that is better than failing compilation completely.

[cbmarcum]

@Vampire This looks good!. I sent you a PR with a test to go along with this if you want or I can add it later.
I'll wait to find out before running the workflows.

[jonnybot0]

I'm inclined to think that we should just update Groovy, since a fix got upstreamed reasonably quickly and backported to the 4.x branch (apache/groovy@2fc8212).

Also, if browser forwarding isn't going to work, maybe compilation failure is the right place for this to fail, rather than the wrong one? I'd rather catch the problem there than at runtime.