ARROW-7708: [Developer][Release] Include PARQUET issues in release ch…

…angelogs by scraping git history

I wrote logic in dev/release/changelog.py that does the following:

* Gets commit descriptions between the last major release and the new release version from `git log`
* Uses the `$PROJECT-$key` strings to select from parquet-cpp issues returned by the JIRA API
* These "Resolved as of the last Arrow release" Parquet issues are included in the changelog

I updated the changelogs back to 0.12.0 which was the first major release (I think) after the monorepo merge was completed. It might be nice to also update the changelogs on the website

Closes #6722 from wesm/ARROW-7708

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>