nuget packages have their own version numbers independent of the framework. Most packages support multiple frameworks, so you need to pick a version that supports your framework version (3.1 or netstandard 2.0/2.1)
for Microsoft.EntityFrameworkCore.SqlServer you will see the last version to support 3.1 was 5.0.17
note: 3.1 support ends in December, so you should start porting to net 6 soon.