How to reproduce:
1. Create a new project using the default 3D template
2. Import the 'Visual Scripting' package
3. In the Project Window, create a new Script Graph (Create->Visual Scripting->Script Graph)
4. Open the created Asset (Double-click)
5. In the Build Settings, switch to the WebGL Platform
6. Make a build
7. Wait for the process to finish
8. Inspect the Console Window

Expected result: The Player is built successfully
Actual result: Build fails as multiple "The type or namespace name 'ReadOnlySpan<>' does not exist in the namespace 'System'" errors are thrown

Reproducible with: 2021.2.0a18, 2021.2.0b1
Not reproducible with: 2021.1.13f1, 2021.2.0a17
Couldn't test with: 2018.4.36f1, 2019.4.28f1, 2020.3.13f1 (Visual Scripting package is not available in Package Manager)

Note:
- Errors cannot be cleared after the Build