"o3": New single label for O3 / OpenMRS 3 / 3.0 / 3.x topics

Quick note that I have amalgamated all recent-ish O3-related posts with the label “o3” - you can see them all here: Topics tagged o3

I did this for simplicity - we had four tags: 3x, 3.x, openmrs-3.0, and o3. (FWIW, o3 also matches the “O3-###” format of the O3 Jira project.)

We can always change the label later if we want but at least for now everything should be in “one place”.