Remove Upper Version Limit on Rails Dependencies

[adrienpoly]

Having an upper limit on the Rails version is now discouraged by the Rails core team, as discussed here: https://github.com/svenfuchs/rails-i18n/pull/1130#discussion_r1808493332

Setting a strict upper bound prevents users from upgrading Rails on day one of a new release. It also blocks contributors from easily testing compatibility without resorting to forking by simply bundling the gem and running the test suite against the latest Rails version.

I’d recommend removing the upper version restriction in the gemspec

Once a new Rails release is available and added to the CI matrix, you can update the compatibility table accordingly in the readme and officially support it. This approach allows early adopters to provide feedback on any potential issues, improving iteration speed without blocking upgrades.