Switch the openapi-codegen to our docker image

[svix-mman]

Using the docker image from svix/openapi-codegen#82
This results in the identical code.

TODO

• Make script more robust (run_generate is very hacky)
• Somehow run each step in parallel
• Maybe re-write the regen_opanpi.sh as a python script
• Wait for these PRs to merge, and then rebase on main
  • csharp: Add codegen templates #1773
  • java: Pull in change from latest codegen #1774
  • kotlin: Pull in change from latest codegen #1775
  • go: Pull in change from latest codegen #1776
  • ruby: Pull in change from latest codegen #1777

[svix-mman]

CI is broken because it's trying to pull an image we have not yet published
CI is slow because I added a docker build command that take 10-15 minutes
Once I merge svix/openapi-codegen#82 this will simply be a docker pull