Update master Branch to main

[mliberatore]

Engineering Task

Change the name of the default branch from master to main.

Constraints and Assumptions

This should be fairly straightforward (Settings > Branches > Default Branch) but think of any implications for clients of PinpointKit who e.g. may be using master in their Podfiles, and consider adding a note in the README about the change. Also, we’ll need to update the README which refers to master regarding the git submodule instructions.

Design and Product Information

N/A