How to reproduce:
1. Open project "Empty Unity Project.zip"
2. Go to Unity>Preferences>External Tools and click the "Regenerate project files" button
3. Open the project folder in your file explorer
4. Open "Assembly-CSharp.csproj" with a text editor but not a VS
5. Look at the "<LangVersion>8.0</LangVersion>" version
6. Close the text editor
7. In the Unity Project window create a new C# Script (Right click>Create>C# Script)
8. Again open "Assembly-CSharp.csproj" with a text editor but not VS
9. Look at the "<LangVersion>latest</LangVersion>" version
10. If the version didn't change go to the 7th step
11. Again go to Unity>Preferences>External Tools and click the "Regenerate project files" button
12. Again open "Assembly-CSharp.csproj" with a text editor but not VS
13. Look at the "<LangVersion>8.0</LangVersion>" version

Expected result: Lang version should be 8.0 (or different in other versions)
Actual result: Lang version is "latest"

Reproducible with: 2.0.11 (2020.1.0a21, 2020.3.20f1, 2021.1.25f1, 2021.2.0b16, 2022.1.0a12)
Not reproducible with: 2.0.11 (2019.4.31f1)
Could not test with: 2020.1.0a20 ("Regenerate project files" button wasn't appearing)