Rename heartbeats to cron check-ins

[unflxw]

See https://github.com/appsignal/appsignal-ruby/pull/1115.

Rename the existing heartbeat functionality to cron check-in. To only emit a single deprecation warning at compile-time, rather than many at run-time, use the built-in @deprecated Elixir functionality. The deprecation warning is not itself under test, but the forwarding of method calls from the old names to the new names is.

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