Schema: TargetOrImportType causes validation failures in otherwise fine .csproj files

https://github.com/dotnet/msbuild/blob/f93b24b5a88a37a3b6b122c7f65f910fcc2ba2f0/src/MSBuild/MSBuild/Microsoft.Build.Core.xsd#L12


Adding a `` or `` removes the warning, but these are not required for most .csproj files, so this should be marked as an optional in the sequence.

## Reproduce
VSCode w/ XML extension

Sample simple .csproj generated with `dotnet new classlib`
```xml

net7.0
enable
enable

```

And vscode settings:
```json
{
"xml.fileAssociations": [
{
"pattern": "**/*.csproj",
"systemId": "https://raw.githubusercontent.com/dotnet/msbuild/main/src/MSBuild/Microsoft.Build.xsd"
}
],
"xml.validation.namespaces.enabled": "onNamespaceEncountered"
}
```