Change the way the BIDS spec changelog is created.

[sappelhoff]

Working on #1836 has revealed that our current way to generate the changelog is error prone and suboptimal. The software we've been using has been looking for a new maintainer for some years, and due to an issue for our particular setup, we have furthermore pinned a version that is highly outdated.

Next to these technical issues, the BIDS maintainers believe that a more carefully curated changelog would be beneficial for the BIDS community trying to keep up with changes in BIDS.

Potential options are:

• https://github.com/twisted/towncrier

... please chime in if you have ideas and/or concrete suggestions on how we should generate the changelog for the BIDS specification in the future.

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Another issue with the current changelog generator is, that our repo is getting so big, that we run into API limitations:

Octokit::TooManyRequests GET https://api.github.com/repos/bids-standard/bids-specification/issues/248/events?per_page=100: 403 - You have exceeded a secondary rate limit. Please wait a few minutes before you try again. If you reach out to GitHub Support for help, please include the request ID 5840:CDA1:20E10D4:3B49182:6659D512. // See: https://docs.github.com/free-pro-team@latest/rest/overview/rate-limits-for-the-rest-api#about-secondary-rate-limits; sleeping for 660s...