This commit moves the gradle wrapper jar file to a hidden directory, so
that it does not clutter the top level names seen when doing an ls in
the project. The actual jar file is never manually edited, and only
changed by running `./gradlew wrapper ...` so it is not important for
this directory to be "visible".
This commit gives us Gradle 4.7 which brings with it official JDK 10
support (which we are already using as our minimum compiler version) and
support for JDK 11 early-access releases (previous versions of Gradle
will not even start on JDK 11).
We have agreed to introduce the Gradle wrapper to simplify workflows for
developers, and managing infrastructure (e.g., CI, release builds, etc.)
as well as consideration for the fact that other projects in our stack
use Gradle and do not necessarily want to be tied to our Gradle version.
Relates #28065
