Update and pin the `jwt` gem to ~> 1.5.6

What does this MR do?

Changelog can be found at https://github.com/jwt/ruby-jwt/blob/master/CHANGELOG.md

• Specific changes from 1.5.4 to 1.5.5 can be found at: https://github.com/jwt/ruby-jwt/compare/v1.5.4...v1.5.5. Please note that there was one bug introduced by this version, which was solved in the 1.5.6 (https://github.com/jwt/ruby-jwt/pull/166) version (the one we're updating to here).

• Specific changes from 1.5.5 to 1.5.6 can be found at: https://github.com/jwt/ruby-jwt/compare/v1.5.5...v1.5.6

Are there points in the code the reviewer needs to double check?

You can look at the full 1.5.4 -> 1.5.6 diff: https://github.com/jwt/ruby-jwt/compare/v1.5.4...v1.5.6

Does this MR meet the acceptance criteria?

• Changelog entry added
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27488 (closed)