Feature ‘using declarations’ is not available in C# 7.3. Please use language version 8.0 or greater – Error on one machine but works on another

I received the same error, but I had simply forgotten to include the

<LangVersion>8.0</LangVersion>

attribute in ALL the .csproj files in the solution.
The following is my current c# 8 setup:

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <LangVersion>8.0</LangVersion>
    <Nullable>enable</Nullable>
    <NullableContextOptions>enable</NullableContextOptions>
  </PropertyGroup>

I found the following documents to be the most helpful when migrating from core 2.2 to 3.x:

MSDN 2.2 -> 3.0

MSDN 3.0 -> 3.1