Proxy fails when using HTTP CONNECT (E.G. HTTPS via a HTTP only proxy)

release-please fails when trying to connect to a HTTP only proxy via HTTP CONNECT
See:
https://github.com/googleapis/release-please/blob/08331fa555ed482f675fb3e9dd5b0a5296e200a3/src/github.ts#L226-L231

It would probably make sense just to take the whole proxy URL (E.G. http://my-proxy:80) and apply it to the HTTP agent instead of making assumptions about the users environment?

Happy to raise a PR for this 🙂

Note: This would also require an update in the GitHub action to stop it splitting the proxy input and would probably therefore be a breaking change for both release-please and the action.

#### Environment details

- OS: Ubuntu 22.04 x64 and Windows 10 x64
- Node.js version: 20.12.1
- npm version: 10.5.1
- `release-please` version: 16.10.1

#### Steps to reproduce

1. Use a proxy value such as `my-proxy:80` which only accepts `http` connections
2. Get an error when release-please incorrectly tries to use `https` connection to the proxy.