fly: Update heroku images to `heroku/builder:22`

```
#######################################################################

WARNING: This builder image (heroku/buildpacks:20) is deprecated,
since it uses legacy shimmed classic Heroku buildpacks, rather than
Heroku's next-generation Cloud Native Buildpacks.

As such, this image is no longer supported and will soon stop receiving
security updates.

Please switch to one of our newer 'heroku/builder:*' builder images,
such as 'heroku/builder:22':
https://github.com/heroku/cnb-builder-images#available-images

If you are using the Pack CLI, you will need to adjust your '--builder'
CLI argument, or else change the default builder configuration:
https://buildpacks.io/docs/tools/pack/cli/pack_config_default-builder/

If you are using a third-party platform to deploy your app, check their
documentation for how to adjust the builder image used for your build.

#######################################################################
```

Search the repository for references to `heroku/buildpacks:20` and identify the Fly deployment configuration that uses it. Update the relevant builder image reference to `heroku/builder:22`, then verify that the configuration no longer uses the deprecated image.

Written by the indexing model from the issue text.