Move 2.0 out of src folder and rename to 2.x

[nickevansuk]

Move 2.0 out of /src/ folder into /versions/ folder in https://github.com/openactive/data-models and rename it to 2.x, to allow it to be referenced externally via GitHub pages at:

https://www.openactive.io/data-models/versions/2.x/examples/event_example_1.json

instead of

https://www.openactive.io/data-models/src/2.0/examples/event_example_1.json

Following our previous conversation about not maintaining minor versions of the spec, and only cloning the models directory for breaking changes (e.g. 3.x)

Also keeps it nicely separated from the code :)