Add Oracle Database 23ai Free "latest" project (#548)

Remove the OracleDatabase/23.3.0-Free and OracleDatabase/23.8.0-Free
projects.

Add an OracleDatabase/23ai-Free project that installs the latest
available version of Oracle Database 23ai Free by default, but can be
configured to install any available version by:

1. Setting the VM_DB_VERSION environment variable, or
2. Updating the VM_DB_VERSION parameter in the .env.local or .env
   file, or
3. Modifying the VM_DB_VERSION parameter in the Vagrantfile

Implementation based on the discussion in #547.

README.md documents the new functionality.

Tested with the VirtualBox and libvirt providers.

Closes #547

Signed-off-by: Paul Neumann <[email protected]>

Author: PaulNeumann