ci: fix build warnings

Author: egil

src/TimeProviderExtensions/System.Threading/PeriodicTimerPort.cs

@@ -16,8 +16,7 @@ namespace TimeProviderExtensions;
 /// <remarks>
 /// This timer is intended to be used only by a single consumer at a time: only one call to <see cref="WaitForNextTickAsync" />
 /// may be in flight at any given moment.  <see cref="Dispose"/> may be used concurrently with an active <see cref="WaitForNextTickAsync"/>
-/// to interrupt it and cause it to return false. Similarly, <see cref="Period"/> may be used concurrently with a consumer accessing
-/// <see cref="WaitForNextTickAsync"/> in order to change the timer's period.
+/// to interrupt it and cause it to return false.
 /// </remarks>
 internal sealed class PeriodicTimerPort : IDisposable
 {

src/TimeProviderExtensions/System.Threading/TimeProviderPeriodicTimerExtensions.cs

@@ -40,7 +40,7 @@ public static PeriodicTimerWrapper CreatePeriodicTimer(this TimeProvider timePro
     /// may be in flight at any given moment. <see cref="System.Threading.PeriodicTimer.Dispose()"/> may be used concurrently with an active <see cref="System.Threading.PeriodicTimer.WaitForNextTickAsync"/>
     /// to interrupt it and cause it to return false.
     /// </remarks>
-    /// <returns>A new <see cref="TimeProviderExtensions.PeriodicTimer"/>. Note, this is a wrapper around a <see cref="System.Threading.PeriodicTimer"/>.</returns>
+    /// <returns>A new <see cref="PeriodicTimer"/>.</returns>
     public static System.Threading.PeriodicTimer CreatePeriodicTimer(this TimeProvider timeProvider, TimeSpan period)
         => new PeriodicTimer(period, timeProvider);
 #endif

src/TimeProviderExtensions/TimeProviderExtensions.csproj

@@ -24,6 +24,8 @@
 
 	<PropertyGroup>
 		<TargetFrameworks>net8.0;net6.0;netstandard2.0</TargetFrameworks>
+		<!-- Microsoft.Bcl.TimeProvider 8.0.0-preview.4.23259.5 doesn't support netcoreapp3.1 and has not been tested with it. -->
+		<SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings>
 		<ImplicitUsings>enable</ImplicitUsings>
 		<Nullable>enable</Nullable>
 		<Deterministic>true</Deterministic>