Stop testing Rails main under Ruby 3.1

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What are you trying to accomplish?

CI jobs are failing because Rails main is now at v8.0, which has dropped support for Ruby 3.1. This PR removes our build matrix entry for Rails main + Ruby 3.1 to avoid failures.

Integration

No changes necessary in production - this is a CI-only change.

Risk Assessment

• Low risk the change is small, highly observable, and easily rolled back.
• Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
• High risk changes are those that could impact customers and SLOs, low or no test coverage, low observability, or slow to rollback.

Accessibility

• No new axe scan violation - This change does not introduce any new axe scan violations.

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Latest commit: 115d667

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