Unify around a single HTTP library

[JamieMagee]

Code improvement description

Currently Dependabot uses four different HTTP libraries:

• http
• excon
• httparty
• faraday

Only two of these are used directly:

• http in Dependabot::ApiClient
• excon everywhere else

The other two are used by API clients:

• faraday by octokit
  • common does have a direct dependency on faraday to implement Dependabot::Clients::GithubWithRetries
• httparty by gitlab

It seems like we're stuck with faraday and httparty no matter what. But I think we could easily get rid of http.

Related: https://honeyryderchuck.gitlab.io/2023/10/15/state-of-ruby-http-clients-use-httpx.html