This change fixes an issue where the results of the Quarkus tests where
not included in the aggregated JaCoCo test report, due to it not using
the official Gradle JaCoCo plugin.

This change defines a new configuration that exposes the execution data
generated by Quarkus to the aggregation plugin.

This change updates the build script to fix the aggregation step for the
JaCoCo coverage reports that are submitted to Codecov. Previously, not
all modules were properly included in the report.

This change updates the project dependencies to follow the latest
available version where possible.

This change upgrades the Gradle build system to version 7.1.

This change updates the Gradle build configuration to use Jacoco 0.8.7,
which is necessary for Kotlin 1.5.0 to work nicely with Jacoco.

This change updates the project structure to become flattened.
Previously, the simulator, frontend and API each lived into their own
directory.

With this change, all modules of the project live in the top-level
directory of the repository. This should improve discoverability of
modules of the project.