Clean up test skip conditions

[Tratcher]

This is a general cleanup of skip conditions and OS version checks

• Use the new MaximumOSVersion attribute and remove a custom local version.
• Switch from OSSkipCondition to MinimumOSVersion, I'll send an extensions PR to remove version check support from OSSkipCondition.
• Remove checks for WindowsVersions.Win2008R2, it's the same value as Win7. I'll send an extensions PR to remove Win2008R2.
• UnFlaky'd the HttpSys Https tests. The linked issue didn't give any details for why they were marked Flaky and the only linked builds have expired.

[Tratcher]

Found a bug in my last MaximumOSVersionAttribute change that copied a Version object. Apparently the value for unspecified segments is -1, which the constructor refuses.

[xUnit.net 00:00:00.68] Microsoft.AspNetCore.Server.Kestrel.Tests: Exception thrown during theory discovery on 'Microsoft.AspNetCore.Server.Kestrel.Tests.KestrelConfigurationBuilderTests.DefaultConfigSectionCanSetProtocols_MacAndWin7'; falling back to single test case.
System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
 ---> System.ArgumentOutOfRangeException: Version's parameters must be greater than or equal to zero. (Parameter 'build')
   at System.Version..ctor(Int32 major, Int32 minor, Int32 build)
   at Microsoft.AspNetCore.Testing.MaximumOSVersionAttribute..ctor(OperatingSystems targetOS, Version maxVersion, OperatingSystems currentOS, Version currentVersion)
   at Microsoft.AspNetCore.Testing.MaximumOSVersionAttribute..ctor(OperatingSystems operatingSystem, String maxVersion)
   --- End of inner exception stack trace ---
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
   at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture)
   at System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes)
   at Xunit.Sdk.ReflectionAttributeInfo.Instantiate(CustomAttributeData attributeData) in C:\Dev\xunit\xunit\src\xunit.execution\Sdk\Reflection\ReflectionAttributeInfo.cs:line 136
   at Xunit.Sdk.ReflectionMethodInfo.GetCustomAttributes(MethodInfo method, Type attributeType, AttributeUsageAttribute attributeUsage) in C:\Dev\xunit\xunit\src\xunit.execution\Sdk\Reflection\ReflectionMethodInfo.cs:line 100
   at Xunit.Sdk.ReflectionMethodInfo.GetCustomAttributes(MethodInfo method, String assemblyQualifiedAttributeTypeName) in C:\Dev\xunit\xunit\src\xunit.execution\Sdk\Reflection\ReflectionMethodInfo.cs:line 87
   at Xunit.Sdk.ReflectionMethodInfo.GetCustomAttributes(String assemblyQualifiedAttributeTypeName) in C:\Dev\xunit\xunit\src\xunit.execution\Sdk\Reflection\ReflectionMethodInfo.cs:line 80
   at Microsoft.AspNetCore.Testing.TestMethodExtensions.EvaluateSkipConditions(ITestMethod testMethod)
   at Microsoft.AspNetCore.Testing.ConditionalTheoryDiscoverer.CreateTestCasesForDataRow(ITestFrameworkDiscoveryOptions discoveryOptions, ITestMethod testMethod, IAttributeInfo theoryAttribute, Object[] dataRow)
   at Xunit.Sdk.TheoryDiscoverer.Discover(ITestFrameworkDiscoveryOptions discoveryOptions, ITestMethod testMethod, IAttributeInfo theoryAttribute) in C:\Dev\xunit\xunit\src\xunit.execution\Sdk\Frameworks\TheoryDiscoverer.cs:line 238

[jkotalik]

Lol this was a thing.