Move from gableroux/unity3d to unityci/editor image

Breaking changes

• We're now using unityci/editor image instead of gableroux/unity3d in order to support better docker image versioning and introduce some fixes.
• This also means that licenses have to be reactivated when upgrading.
• Unity versions lower than 2018.2 are no longer supported (recommendation is to upgrade).

Changes

• Activation will now work across all unity versions within all editor images.

Credits

Thanks to @BLaZeKiLL for converting the action to use the new images.