Automate script tools/convert-shared-credential-to-legacy-format.rb

[parkerbxyz]

Run ruby tools/convert-shared-credential-to-legacy-format.rb --verify
  ruby tools/convert-shared-credential-to-legacy-format.rb --verify
  shell: /usr/bin/bash -e {0}
ERROR: websites-with-shared-credential-backends.json is not up-to-date. Please run this script again and commit the changes.
Error: Process completed with exit code 1.

Please run tools/convert-shared-credential-to-legacy-format.rb and commit the result of that. (Sorry about this! We should automate that somehow.)

Originally posted by @rmondello in #714 (comment)

I can automate this. Feel free to assign this issue to me and I'll start working on a pull request.