Bump Ruby checkout to 3.2

[reese]

Fully unblocks #399

On the tin -- this bumps the checkout to a7d467a792c644a7260d6560ea2002fdb8ff6de3, which is the most recent revision on the ruby_3_2 branch. There are a few changes to support this:

• The value of Qnil was updated internally, so I've updated to reflect that
• Ripper now requires Bison 3, so I updated the github action
• Arg forwarding's representation in Ripper was changed (although I didn't look up the exact commits there), I imagine to support anonymous blocks and so on, so I at least updated the deserialization to work for our existing use cases