Updated contributor branch references - 2.0 no longer maintained

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.1+
| Fixed tickets | N/A

The current documentation refers throughout to making bugfix and corrections against the 2.0 branch, however this is no longer maintained. Updated with references to 2.1 to make this more obvious to new contributors.

Obviously this would then need to be repeated each time you drop support for an old version - I'm not sure if you want to go down that route. Alternatively perhaps it would be enough to leave the references in the examples but provide an obvious band on the code and documentation pages clearly showing the oldest maintained version number?