Downgrade gradle version to support testing on windows server 2016

[donoghuc]

When gradle was upgraded in #17947 the windows 2016 acceptance tests started to fail due to gradle dropping support for that platform. #18209 (comment)

We want to keep the forward compatability changes in #17947 and downgrade to the last version of gradle that supports windows 2016 (probably 8.11.1 according to gradle/gradle#31939 (comment)).