arrow

ARROW-15692: [Dev] Update release scripts to use default branch

## Overview

This pull request is part of a larger effort to rename the default branch from `master` to `main`. In order to enable a smooth transition and streamline any future renaming tasks, the default branch name of `apache/arrow` is computed in the release scripts, instead of hardcoded.

## Implementation
1. Added a script, `git-vars.sh` that computes the `DEFAULT_BRANCH` of a Git repository. Calculations for other required Git variables, in the future, can be added to this script.
2. For the following release scripts, hardcoded references to 'master' have been replaced with the value of `DEFAULT_BRANCH`:
    - `01-prepare.sh`
    - `post-12-bump-versions.sh`
    - `post-04-website.sh`
    - `post-09-docs.sh`
3. The changes to `01-prepare.sh` and 'post-12-bump.sh` are required for renaming the default branch name for `apache/arrow`.
4. To prepare for the default branch name change of the `arrow-site` repository, references to `master` are replaced with generic references to the default branch in`post-04-website.sh` and `post-09-docs.sh`.

## Testing
1. The Release Tests in `ci/scripts/release_test.sh` pass in the CI job.

## Notes
1. Thank you @kevingurney and @sreeharihegden, for helping me with this pull request!

Closes #12489 from lafiona/ARROW-15689

Lead-authored-by: Fiona La <fionala@mathworks.com>
Co-authored-by: lafiona <fionala7@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>