Feel free to make PRs to the README.
Should also perhaps add a link to the small grants program.
Note that fixing regressions, e.g. those from 1.12, is vastly easier than writing the code in the first place.
It’s mostly straightforward mechanical changes. Because it’s also a regression, you can also compare older and newer Julia versions side by side to better understand the changes.
It really isn’t hard. But it isn’t a mentorship program, so you shouldn’t expect any guidance aside from what you can get asking in open channels like discourse, slack, or zulip.