Upgrade to Ruby 3.4

[etagwerker]

Context

We are currently using Ruby 2.7: https://github.com/fastruby/audit/blob/main/Gemfile#L11-L17

Expected

We would like the application to use Ruby 3.4.

Resources

It might be connected to these issues:

• Upgrade to Rails 7.0 #63
• Upgrade to Rails 7.1 #64
• Upgrade to Rails 7.2 #65
• Upgrade to Rails 8.0 #66