[Dev] Replace hard-coded string "master" with "main" in dev/archery/archery/crossbow/core.py after default branch migration

[asfimport]

#14033 uses several heuristics to determine the default Git branch name for the upstream apache/arrow project. When the heuristics fail to detect the default branch name, the strategy used in #14033 currently falls back to using the hard-coded branch name "master".

After #14033 is merged and after the upstream apache/arrow GitHub repository migrates to using "main" as the default branch name, the hard-coded string "master" should be replaced with "main" inside of the implementation of the computed property "default_branch_name" of the "Repo" class in dev/archery/archery/crossbow/core.py.

A similar instance of hard-coded "master" is used in [dev/merge_arrow_pr.py](https://github.com/apache/arrow/pull/14533/files#diff-4aea0167b68c23bb8725640aec90bfc1d4caed14e9c21e1b64d65a54f9b47e63) and it needs to be updated as well.

Reporter: Kevin Gurney / @kevingurney

_{Note: This issue was originally created as ARROW-18011. Please see the migration documentation for further details.}