Explicitly use bundler 2.7 on Ruby 4 CI #571

Job	Run time
lint / lint	47s
docker_build / build	25s
test / set_matrix	3s
test / ruby 3.4 \| rails 8.1.1	4m 4s
test / ruby 4.0 \| rails 8.1.1 \| Ruby 4	3m 51s
test / ruby 3.4 \| rails 8.0.4 \| esbuild	4m 10s
test / ruby 3.3 \| rails 8.0.4 \| Rails 8.0 + Ruby 3.3	4m 17s
test / ruby 3.4 \| rails 7.2.3 \| Rails 7.2 + Propshaft, esbuild	4m 22s
test / ruby 3.4 \| rails 8.1.1 \| esbuild	4m 11s
test / ruby 3.4 \| rails 8.1.1 \| API	2m 20s
test / ruby 3.4 \| rails 8.0.4 ViewComponent 3	4m 3s
test / ruby 3.2 \| rails 8.0.4 \| Solr 8.11.2 and Ruby 3.2	3m 37s
	36m 10s

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