fix: deprecate master branch and rename as main

[Jason Schmidt]

Is your feature request related to a problem? Please describe.
We need to rename our "master" branch to "main" to conform with new best practices for inclusion; this should be undertaken with some caution in that we do have a few areas where the branch name is used (for example, Jenkins).

Describe the solution you'd like
Rename master -> main

Describe alternatives you've considered
None

Additional context
None