Update excon to v0.99.0

What does this MR do and why?

This update is needed to fix test failures on Ruby 3.1.4 due to updates in Ruby's URI parsing: https://github.com/excon/excon/pull/808.

• Gem diff: https://my.diffend.io/gems/excon/0.90.0/0.99.0
• Source diff: https://github.com/excon/excon/compare/v0.90.0...v0.99.0

How to set up and validate locally

1. Set up object store with S3: https://docs.gitlab.com/ee/administration/object_storage.html
2. Attempt to upload attachments

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.