Rename system test "deployAll" deployment to "demonstration"

### User Story

As a user of Sleeper, I want the demonstration deployment to be named intuitively, so that we don't confuse it with the system tests.

### Description / Background

The scripts under `scripts/test/deployAll` are a "demonstration" deployment of Sleeper, that comes with an ECS cluster for random data generation.

We'd like to ensure that this is named consistently everywhere. We'd like to remove uses of "system test" to refer to this, so that that should just refer to the Maven-based system test suite.

### Technical Notes / Implementation Details

This includes the documentation, the `scripts/test/deployAll` folder name, and the ECS cluster for data generation (which is currently called the system test cluster).

The data generation cluster is currently deployed in `SystemTestClusterStack`.

Several of the modules under the system-test parent module are used for both system tests and the demonstration deployment.

`SystemTestProperties` could be called `DemonstrationInstanceProperties`. The version of this class that is used in system tests is `SystemTestStandaloneProperties`.

The demonstration instance properties are currently copied from a template in `scripts/test/deployAll/system-test-instance.properties.template`. This could be renamed.

Start by reviewing scripts/test/deployAll, SystemTestClusterStack, and the system-test parent module to map uses of the old name. Check SystemTestProperties, SystemTestStandaloneProperties, and system-test-instance.properties.template, along with the documentation. Done means the demonstration deployment, ECS cluster, classes, template, folder, and documentation use consistent names without conflating them with Maven system tests.

Written by the indexing model from the issue text.