OPT: Use .ruby-version instead of hardcoded version on Gemfile #526

Job	Run time
Build Image / Cypress	26s
Build Image / Development	24s
Build Image / Production	12s
Build Image / Test	13s
Build Image / Preview	12s
Lint / Brakeman	21s
Lint / Rubocop	25s
Lint / SCSSLint	21s
Lint / Bundler Audit	20s
Lint / Reek	19s
Tests / Backend	49s
Preview / Deploy	20s
Tests / End-to-end - Frontend (chrome)	1m 55s
Lint / ESLint - Tests	39s
Tests / End-to-end - Frontend (firefox)	1m 35s
Lint / ESLint	39s
Tests / End-to-end - Admin (chrome)	2m 41s
Tests / End-to-end - Admin (firefox)	2m 42s
	14m 33s

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