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

https://github.com/apache/arrow/pull/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 https://github.com/apache/arrow/pull/14033 currently falls back to using the hard-coded branch name "master".

After https://github.com/apache/arrow/pull/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` and it needs to be updated as well.