chore: convert root template to .heex #5303

Job	Run time
file_changes	5s
Build Docker image	7m 41s
Build application and assets	6m 31s
Linting / Elixir	3m 39s
Formatting / Elixir	3m 27s
Linting / TypeScript	39s
Formatting / JavaScript & TypeScript	42s
Unit tests / JavaScript / Mocha	51s
Linting / JavaScript	32s
Unit tests / JavaScript & TypeScript / Jest	2m 6s
Linting / CSS	23s
Type checks / Elixir	3m 55s
Type checks / TypeScript	33s
Unit tests / Elixir / mix coveralls.html	57s
	32m 1s

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