[🚀 Feature]: [dotnet] Align and simplify root namespace, who is OpenQA?

### Description

This is a continuation of https://github.com/SeleniumHQ/selenium/issues/15791

Given that soon we will have
- `Selenium.WebDriver` nuget package (now)
- `Selenium.WebDriver.dll` (in v5?)

```csharp
using var driver = new OpenQA.Selenium.WebDriver.Chrome.ChromeDriver();
```

Then in .NET world the question is **who is `OpenQA`**? Seems only `.NET` and `Java` has this prefix as an organization.

### Have you considered any alternatives or workarounds?

Yes, be more predictable.

```
dotnet new console
dotnet add package selenium.webdriver
```

```csharp
using var driver = new Selenium.WebDriver.Chrome.ChromeDriver();
```

Done! No mistic `OpenQA`.