chore: convert root template to .heex #5304

Job	Run time
Build application and assets	6m 35s
file_changes	5s
Build Docker image	7m 27s
Linting / CSS	25s
Linting / TypeScript	38s
Unit tests / JavaScript / Mocha	47s
Linting / Elixir	3m 31s
Unit tests / JavaScript & TypeScript / Jest	2m 4s
Linting / JavaScript	25s
Type checks / Elixir	4m 2s
Formatting / Elixir	3m 11s
Formatting / JavaScript & TypeScript	40s
Type checks / TypeScript	39s
Unit tests / Elixir / mix coveralls.html	1m 3s
	31m 32s

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