Kindly remove .NET 3.x

[cnayan]

As .NET Core 3.x has reached end of life, kindly remove it from the lib.

[pakrym]

.NET Core 3 doesn't cause any maintenance issues yet, so I'd like to keep it until it does.

[eXpl0it3r]

With ubuntu-latest (i.e. ubuntu-24.04), there's now an issue with resolving the correct ICU for .NETCore 3 in the GitHub pipeline. ubuntu-22.04 still works correctly. Not sure if one would need to find the correct ICU version and manually install it in the process.

GitHub Action failure:

Starting test execution, please wait...
[xUnit.net 00:00:00.00] xUnit.net VSTest Adapter v2.4.5+1caef2f33e (64-bit .NET 8.0.12)
A total of 1 test files matched the specified pattern.
Process terminated. Couldn't find a valid ICU package installed on the system. Set the configuration flag System.Globalization.Invariant to true if you want to run with no globalization support.
   at System.Environment.FailFast(System.String)
   at System.Globalization.GlobalizationMode.GetGlobalizationInvariantMode()
   at System.Globalization.GlobalizationMode..cctor()
   at System.Globalization.CultureData.CreateCultureWithInvariantData()
   at System.Globalization.CultureData.get_Invariant()
   at System.Globalization.CultureInfo..cctor()
   at System.String.ToUpperInvariant()
   at System.Diagnostics.Tracing.EventSource.GetGuid(System.Type)
   at System.Diagnostics.Tracing.EventSource..ctor(System.Diagnostics.Tracing.EventSourceSettings, System.String[])
   at System.Diagnostics.Tracing.EventSource..ctor()
   at Microsoft.VisualStudio.TestPlatform.CoreUtilities.Tracing.TestPlatformEventSource..ctor()
   at Microsoft.VisualStudio.TestPlatform.CoreUtilities.Tracing.TestPlatformEventSource..cctor()
   at Microsoft.VisualStudio.TestPlatform.CoreUtilities.Tracing.TestPlatformEventSource.get_Instance()
   at Microsoft.VisualStudio.TestPlatform.TestHost.Program.Main(System.String[])
Testhost process for source(s) '/home/runner/work/jab/jab/src/Jab.FunctionalTests.Attributes/bin/Debug/netcoreapp3.1/Jab.FunctionalTests.Attributes.dll' exited with error: Process terminated. Couldn't find a valid ICU package installed on the system. Set the configuration flag System.Globalization.Invariant to true if you want to run with no globalization support.
   at System.Environment.FailFast(System.String)
   at System.Globalization.GlobalizationMode.GetGlobalizationInvariantMode()
   at System.Globalization.GlobalizationMode..cctor()
   at System.Globalization.CultureData.CreateCultureWithInvariantData()
   at System.Globalization.CultureData.get_Invariant()
   at System.Globalization.CultureInfo..cctor()
   at System.String.ToUpperInvariant()
   at System.Diagnostics.Tracing.EventSource.GetGuid(System.Type)
   at System.Diagnostics.Tracing.EventSource..ctor(System.Diagnostics.Tracing.EventSourceSettings, System.String[])
   at System.Diagnostics.Tracing.EventSource..ctor()
   at Microsoft.VisualStudio.TestPlatform.CoreUtilities.Tracing.TestPlatformEventSource..ctor()
   at Microsoft.VisualStudio.TestPlatform.CoreUtilities.Tracing.TestPlatformEventSource..cctor()
   at Microsoft.VisualStudio.TestPlatform.CoreUtilities.Tracing.TestPlatformEventSource.get_Instance()
   at Microsoft.VisualStudio.TestPlatform.TestHost.Program.Main(System.String[])
. Please check the diagnostic logs for more information.

Test Run Aborted.