Centralise and improve Rails version handling #1523

Job	Run time
rubocop	13s
test	9s
audit_dependencies	6s
test_generated_apps (defaults, ackama_rails_template.config.yml, yes)	46s
test_generated_apps (all, ci/configs/all.yml, yes)	52s
test_generated_apps (all-typescript, ci/configs/all-typescript.yml, yes)	49s
test_generated_apps (basic, ci/configs/basic.yml)	42s
test_generated_apps (basic-typescript, ci/configs/basic-typescript.yml)	41s
test_generated_apps (github_actions, ci/configs/github_actions.yml, yes)	49s
test_generated_apps (react, ci/configs/react.yml)	46s
test_generated_apps (react-typescript, ci/configs/react-typescript.yml)	49s
test_generated_apps (sidekiq, ci/configs/sidekiq.yml)	51s
test_generated_apps (devise, ci/configs/devise.yml)	48s
test_generated_apps (basic_with_skips, ci/configs/basic.yml, --skip-spring --skip-javascript)	43s
test_generated_apps (bootstrap, ci/configs/bootstrap.yml)	52s
test_generated_apps (bootstrap-typescript, ci/configs/bootstrap-typescript.yml)	51s
test_generated_apps (capistrano, ci/configs/deploy_with_capistrano.yml)	47s
test_generated_apps (ackama_ec2_capistrano, ci/configs/deploy_with_ackama_ec2_capistrano.yml)	49s
	12m 23s

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