Routine releases in Sprint 23

[bassosimone]

This is about preparing a new stable release of ooni/probe-engine as well as a stable release of ooni/probe-cli. We are going to use such releases as the basic building blocks for upcoming probe-desktop and probe-{ios,android} releases.

• Update dependencies
• Update internal/httpheader/useragent.go
• Update version/version.go
• Update internal/resources/assets.go
• Run go generate ./...
• Tag a new version of ooni/probe-engine
• Update again version.go to be alpha
• Create release at GitHub
• Update ooni/probe-engine mobile-staging branch
• Pin ooni/probe-cli to ooni/probe-engine
• Pin ooni/probe-android to latest mobile-staging
• Pin ooni/probe-ios to latest mobile-staging

[bassosimone]

Moving to next sprint and transferring the effort points to #893