Update dependency to fix OOM bug (#6689)

Fixes #6630.

DotNetty has an OOM bug that is fixed in v0.7.5. Two components in Edge Hub have a transitive dependency on DotNetty:
* Microsoft.Azure.Devices.Edge.Hub.Core via Microsoft.Azure.Devices.Client
* Microsoft.Azure.Devices.Edge.Hub.Mqtt via Microsoft.Azure.Devices.ProtocolGateway.Core.

The best approach is for our dependencies to update their reference to DotNetty, then we update our dependencies. In the meantime, we'll make explicit references to override the transitive versions.

I used `dotnet list package --include-transitive` before and after the change to verify that the transitive dependency is replaced by the explicit dependency in the two updated projects, and that transitive packages are at 0.7.5 throughout the rest of Edge Hub. It would work to _only_ add explicit references in Microsoft.Azure.Devices.Edge.Hub.Core since Microsoft.Azure.Devices.Edge.Hub.Mqtt depends on Core but adding explicit references at the site of each dependency should help avoid surprises.

Changes to Edge Hub connectivity can have far-reaching impacts, so I'm running end-to-end, nested end-to-end, connectivity, and long-haul pipelines to test this change.

## Azure IoT Edge PR checklist:

edge-hub/core/src/Microsoft.Azure.Devices.Edge.Hub.Core/Microsoft.Azure.Devices.Edge.Hub.Core.csproj

@@ -20,6 +20,19 @@
     <PackageReference Include="JetBrains.Annotations" Version="2018.3.0" />
     <PackageReference Include="Microsoft.Azure.Devices.Client" Version="1.36.6" />
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
+    <!--
+      We have a transitive dependency on Dotnetty via Microsoft.Azure.Devices.Client. Use
+      explicit references here to override the version and fix a known OOM bug. See
+      https://github.com/Azure/DotNetty/pull/583. If Microsoft.Azure.Devices.Client updates
+      its dependency, we can remove the Dotnetty references.
+    -->
+    <PackageReference Include="DotNetty.Buffers" Version="0.7.5" />
+    <PackageReference Include="DotNetty.Codecs" Version="0.7.5" />
+    <PackageReference Include="DotNetty.Codecs.Mqtt" Version="0.7.5" />
+    <PackageReference Include="DotNetty.Common" Version="0.7.5" />
+    <PackageReference Include="DotNetty.Handlers" Version="0.7.5" />
+    <PackageReference Include="DotNetty.Transport" Version="0.7.5" />
+
   </ItemGroup>
 
   <ItemGroup>

edge-hub/core/src/Microsoft.Azure.Devices.Edge.Hub.Mqtt/Microsoft.Azure.Devices.Edge.Hub.Mqtt.csproj

@@ -11,6 +11,19 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.Azure.Devices.ProtocolGateway.Core" Version="2.0.1" />
     <PackageReference Include="Microsoft.Extensions.Configuration.Binder" Version="5.0.0" />
+    <!--
+      We have a transitive dependency on Dotnetty via Microsoft.Azure.Devices.ProtocolGateway.Core.
+      Use explicit references here to override the version and fix a known OOM bug. See
+      https://github.com/Azure/DotNetty/pull/583. If ProtocolGateway updates its dependency,
+      we can remove the Dotnetty references.
+    -->
+    <PackageReference Include="DotNetty.Buffers" Version="0.7.5" />
+    <PackageReference Include="DotNetty.Codecs" Version="0.7.5" />
+    <PackageReference Include="DotNetty.Codecs.Mqtt" Version="0.7.5" />
+    <PackageReference Include="DotNetty.Common" Version="0.7.5" />
+    <PackageReference Include="DotNetty.Handlers" Version="0.7.5" />
+    <PackageReference Include="DotNetty.Transport" Version="0.7.5" />
+
   </ItemGroup>
 
   <ItemGroup>