Explicitly use bundler 2.7 on Ruby 4 CI #574

Job	Run time
docker_build / build	26s
test / set_matrix	3s
lint / lint	46s
test / ruby 3.4 \| rails 8.1.1	3m 35s
test / ruby 3.4 \| rails 7.2.3 \| Rails 7.2 + Propshaft, esbuild	4m 10s
test / ruby 3.4 \| rails 8.1.1 \| API	2m 15s
test / ruby 3.4 \| rails 8.1.1 \| esbuild	3m 59s
test / ruby 3.2 \| rails 8.0.4 \| Solr 8.11.2 and Ruby 3.2	3m 38s
test / ruby 3.4 \| rails 8.0.4 ViewComponent 3	3m 57s
test / ruby 3.3 \| rails 8.0.4 \| Rails 8.0 + Ruby 3.3	4m 27s
test / ruby 4.0 \| rails 8.1.1 \| Ruby 4	4m 5s
test / ruby 3.4 \| rails 8.0.4 \| esbuild	4m 7s
	35m 28s

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