fix minor warnings (#1278)

Author: SimonCropp

src/Sentry/Envelopes/EnvelopeItem.cs

@@ -68,7 +68,7 @@ private async Task<MemoryStream> BufferPayloadAsync(CancellationToken cancellati
             return buffer;
         }
 
-        private async Task SerializeHeaderAsync(
+        private static async Task SerializeHeaderAsync(
             Stream stream,
             IReadOnlyDictionary<string, object?> header,
             CancellationToken cancellationToken = default)

src/Sentry/Internal/Http/CachingTransport.cs

@@ -53,10 +53,10 @@ public CachingTransport(ITransport innerTransport, SentryOptions options)
 
             _processingDirectoryPath = Path.Combine(_isolatedCacheDirectoryPath, "__processing");
 
-            _worker = Task.Run(CachedTransportBackgroundTask);
+            _worker = Task.Run(CachedTransportBackgroundTaskAsync);
         }
 
-        private async Task CachedTransportBackgroundTask()
+        private async Task CachedTransportBackgroundTaskAsync()
         {
             try
             {