This change updates the Gradle configuration to allow the user to
overide the version number OpenDC will use when producing the build
artifacts.

This change updates the release and publish workflows in GitHub Actions
to ensure that we only publish the artifacts to Maven Central once the
release on GitHub is published.

When a version tag is pushed to GitHub, only a draft release will be
created containing the right artifacts.

This change updates the build script in preparation for the OpenDC v3.0
release changing the version numbers to the appropriate values.

This change removes the distribution conventions used by the root
project of OpenDC. Previously, we used this to build a single
distribution for OpenDC containing the experiment uberjars and scripts
to start OpenDC.

However, with the introduction of the Quarkus-based web server, we have
decided to split releases into (potentially) multiple distributions
(e.g., one for the web server, one for just the web runner, etc.).

Furthermore, the implementation of this convention caused issues with
several other Gradle plugins.

This change moves the version number to the next release, v2.1-rc1.

This change updates the version numbers officially to v2.0 in order to
start the release process.

This change adds support for publishing OpenDC to Maven Central. This
allows other people to develop with OpenDC without needing to build the
entire project themselves.

This change configures the Distribution plugin for the root project and
aggregates the artifacts of the other projects to generate a single
distribution file containing all libraries and binaries.

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.