[SCC-334] Update rails to version 7

[Adnilson]

Description

What: We want to run RMT on Ruby 3.4. However, Ruby 3.4 doesn't support Rails 6.

Why: Rails 6 is last supported on Ruby 3.2. We want to upgrade the framework step by step. So we should migrate Rails to the next major version available on Ruby 3.4.

• Related Issue / Ticket / Trello card: https://jira.suse.com/browse/SCC-334

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Change Type

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• Bug Fix (a non-breaking change which fixes an issue)
• New Feature (a non-breaking change which adds new functionality)
• Documentation Update (a change which only updates documentation)

Checklist

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• I have reviewed my own code and believe that it's ready for an external review.
• I have provided comments for any hard-to-understand code.
• I have documented the MANUAL.md file with any changes to the user experience.
• If my changes are non-trivial, I have added a changelog entry to notify users at package/obs/rmt-server.changes.

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