OpsWorks: cannot connect with Github due to SSH key / RSA issues

Our OpsWorks stacks will no longer deploy. They are failing when trying to connect to GitHub. Here's the error (with my github stuff obfuscated):

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Error executing action `checkout` on resource 'git[Download Custom Cookbooks]'
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Mixlib::ShellOut::ShellCommandFailed
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Expected process to exit with [0], but received '128'
---- Begin output of git ls-remote "git@github.com:********"  ----
STDOUT: 
STDERR: ERROR: You're using an RSA key with SHA-1, which is no longer allowed. Please use a newer client or a different key type.
Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.
 
fatal: Could not read from remote repository.
 
Please make sure you have the correct access rights
and the repository exists.

The repo does exist. And when I try to put in a newer form of RSA key, the OpsWorks control panel rejects it. OpsWorks seems to want an old version of RSA which GitHub will no longer support. We're totally stuck. How do we get out of this?