Deprecate Capistrano deploy method

[tombruijn]

It's using the underlying "notify_of_deploy" behavior, and we should recommend the "revision" config option instead.

We've already deprecated the notify_of_deploy CLI tool: https://docs.appsignal.com/ruby/command-line/notify_of_deploy.html
Read the deploy markers page for more context: https://docs.appsignal.com/application/markers/deploy-markers.html

Part of https://github.com/appsignal/integration-guide/issues/111

Issue affected by this decision: #323

To do

• Print deprecation warning on usage of Capistrano 2 & 3 integrations. Their only purpose should be to send the "notify of deploy" request. And if so, print a deprecation warning when it does so.
• Update Capistrano docs to mention the appsignal_revision option is deprecated. Done in commit e4ac141.