Upgrade gradle from v3.5.1

[tdudgeon]

The latest gradle is at 6.5.1 but we are at 3.5.1.
See how far we can upgrade without major problems.

[tdudgeon]

Gradle upgraded to 4.2.1.
ffff9a7

Trying to upgrade to 4.3.1 breaks the build with an issue that can be found here: https://discuss.gradle.org/t/change-in-copy-method-in-4-3-1/37083.

This change is on the infra_upgrade branch along with other infrastructure upgrades.
The changes have not been tested yet. Just that the app builds OK.

[tdudgeon]

The upgrade was reverted as it caused a strange problem with the cellexecutor container not running correctly. It shut down as soon as it started.
Probably something changed in the build resulting in the jars not being built correctly. Need to re-investigate this later.