Reinstate support for REST Assured now that it supports Groovy 5

[snicoll]

See https://github.com/rest-assured/rest-assured/wiki/ReleaseNotes60 and #1000

[wilkinsona]

I don't want to rush into this and it may be that support for RestClient takes the place of REST Assured support. With the limited resources available to work on REST Docs, I'm not sure it would be wise to support both.