[BUG]: v6.13.0 lower cases all team members

### Expected Behavior

Usernames shouldn't be lowercased, of if they are lowercased consistently lowercased.
v6.13.0 brought in some lowercasing on team members, v6.12.1 was good.

### Actual Behavior

```
# github_team_members.misc_devs has changed
~ resource "github_team_members" "misc_devs" {
id = "123456"
# (2 unchanged attributes hidden)

- members {
- role = "member" -> null
- username = "ChrisSomethingSomething" -> null
}
+ members {
+ role = "member"
+ username = "chrissomethingsomething"
}

# (6 unchanged blocks hidden)
}
```

```
# github_team_members.misc_devs will be updated in-place
~ resource "github_team_members" "misc_devs" {
id = "123456"
# (2 unchanged attributes hidden)

- members {
- role = "member" -> null
- username = "chrissomethingsomething" -> null
}
+ members {
+ role = "member"
+ username = "ChrisSomethingSomething"
}

# (6 unchanged blocks hidden)
}
```

### Terraform Version

OpenTofu v1.12.0
on linux_amd64
+ provider registry.opentofu.org/integrations/github v6.13.0

### GitHub Installation Type

- [x] GitHub.com (Free, Pro, or Team)
- [ ] GitHub Enterprise Server (on-premises)
- [ ] GitHub Enterprise Cloud with Personal Accounts (github.com)
- [ ] GitHub Enterprise Cloud with Managed Users/EMU (github.com)
- [ ] GitHub Enterprise Cloud with Data Residency (*.ghe.com)
- [ ] I don't know

### Affected Resource(s)

- github_team_members

### Terraform Configuration Files

```hcl

```

### Steps to Reproduce

_No response_

### Debug Output

```shell

```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct