Update cimg/ruby Docker tag to v4

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
cimg/ruby	docker	major	3.4.8-browsers → 4.0.1-browsers

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[github-actions]

Container Scanning Status: ✅ Success

ghcr.io/pulibrary/imagecat-rails:pr-326 (debian 13.3)
=====================================================
Total: 0 (HIGH: 0, CRITICAL: 0)

[eliotjordan]

Don't think we're ready for Ruby 4.0 yet.