Elixir CI

ci: add treefmt as code formatting multiplexer, refactor CI to avoid duplication, reorg CI into DevOps workflow #3458

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Run time

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Job	Run time
Test (Elixir 1.16.2 / OTP 26)	1m 20s
Lint (Elixir 1.16.2 / OTP 26)	1m 30s
	2m 50s