Change `signUpErrors.InvalidUserName` to `signUpErrors.InvalidUsername`

[oliver-day]

Description

While creating the named signUpErrors error causes, I got a little too tab happy with GitHub Copilot and did not notice that I created the named InvalidUserName error cause where the "N" in username is capitalized. This named error cause does not follow the convention of other named error causes involving the username value like UsernameIsUnavailable or UsernameIsRequired, where the "n" in username is lowercase.

Update all references of signUpErrors.InvalidUserName to signUpErrors.InvalidUsername for the following:

• [ ] connect > src/features/user/sign-up-error-causes.js (line 20)
• [ ] connect > src/example-components/sign-up-connect/sign-up-controller-component.js (line 90)
• [ ] greenruhm-web > src/features/account-setup/api/check-existing-accounts-integration-test.js (line 173)
• [ ] greenruhm-web > src/features/account-setup/api/create-account-integration-test.js (line 82)
• [ ] greenruhm-web > src/features/account-setup/api/check-existing-accounts.js (lines 39 & 42)

This update will need to be performed in connect first, then we'll need to have a new public release of connect in order for this named error cause to be properly referenced for the updates in greenruhm-web.

[ericelliott]

Good catch. I missed that in the review!