MINOR: [C#] Bump Google.Protobuf and System.Memory in /csharp (#45728)

Bumps [Google.Protobuf](https://github.com/protocolbuffers/protobuf) and System.Memory. These dependencies needed to be updated together.
Updates `Google.Protobuf` from 3.29.3 to 3.30.0
<details>
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<li>See full diff in <a href="https://github.com/protocolbuffers/protobuf/commits">compare view</a></li>
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Updates `System.Memory` from 4.6.0 to 4.5.3

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Authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Signed-off-by: Curt Hagenlocher <[email protected]>

Author: dependabot[bot]

csharp/src/Apache.Arrow.Flight/Apache.Arrow.Flight.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.29.3" />
+    <PackageReference Include="Google.Protobuf" Version="3.30.0" />
     <PackageReference Include="Grpc.Net.Client" Version="2.65.0" />
     <PackageReference Include="Grpc.Tools" Version="2.70.0" PrivateAssets="All" />
     <PackageReference Include="System.Memory" Version="4.6.0" />

csharp/test/Apache.Arrow.Flight.TestWeb/Apache.Arrow.Flight.TestWeb.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.29.3" />
+    <PackageReference Include="Google.Protobuf" Version="3.30.0" />
     <PackageReference Include="Grpc.AspNetCore" Version="2.67.0" />
   </ItemGroup>