[actions] change username for register_devices to optional

[Econa77]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

No longer need to specify a username when using the App Store Connect API.

Description

• Change username to optional.
• related [action] App Store Connect API Key for register_device and register_devices #17426

Testing Steps

[rogerluan]

Nice catch! 🎉 even the description already says it's optional 😊
I checked register_device.rb (singular version) and it is already optional there 👌

Thanks for this contribution!

[joshdholtz]

Ahhh, good find! Thank you so much for the contribution ❤️

[fastlane-bot]

Hey @Econa77 👋

Thank you for your contribution to fastlane and congrats on getting this pull request merged 🎉
The code change now lives in the master branch, however it wasn't released to RubyGems yet.
We usually ship about once a week, and your PR will be included in the next one.

Please let us know if this change requires an immediate release by adding a comment here 👍
We'll notify you once we shipped a new release with your changes 🚀

[fastlane-bot]

Congratulations! 🎉 This was released as part of fastlane 2.172.0 🚀