phx.new - Fix deprecation warning when using Elixir ~> 1.18

[psantos10]

When generating a new application using Elixir 1.18.0-dev, this warning is raised:

Compiling 1 file (.ex)
warning: Mix.Rebar.rebar_cmd/1 is deprecated. Use rebar_args/2 or available?/1 instead
│
275 │ Mix.Rebar.rebar_cmd(:rebar3)
│ ~
│
└─ lib/mix/tasks/phx.new.ex:275:17: Mix.Tasks.Phx.New.rebar_available?/0

Not sure if this is a good practice, but that's I come up with.

[josevalim]

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