MINOR: [C#] Bump Google.Protobuf and System.Memory in /csharp (#45101)

Bumps [Google.Protobuf](https://github.com/protocolbuffers/protobuf) and System.Memory. These dependencies needed to be updated together.
Updates `Google.Protobuf` from 3.29.1 to 3.29.2
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/protocolbuffers/protobuf/commit/233098326bc268fc03b28725c941519fc77703e6"><code>2330983</code></a> Updating version.json and repo version numbers to: 29.2</li>
<li><a href="https://github.com/protocolbuffers/protobuf/commit/17726576c00d4509bc3552ec922de2399f6503fe"><code>1772657</code></a> Automated rollback of commit 23aada230b2478c7a07fe7612489eb8e79b9c379. (<a href="https://redirect.github.com/protocolbuffers/protobuf/issues/19692">#19692</a>)</li>
<li><a href="https://github.com/protocolbuffers/protobuf/commit/8b9d76c458bee6bd9e1d8edacace5b2aa7c5a4c7"><code>8b9d76c</code></a> Export environment variables so bazelisk picks them up (<a href="https://redirect.github.com/protocolbuffers/protobuf/issues/19690">#19690</a>)</li>
<li><a href="https://github.com/protocolbuffers/protobuf/commit/a1c9b6a2dbda0ab13037e60ac0d6edcef93aaab7"><code>a1c9b6a</code></a> Pin staleness check to Bazel 7 (<a href="https://redirect.github.com/protocolbuffers/protobuf/issues/19689">#19689</a>)</li>
<li><a href="https://github.com/protocolbuffers/protobuf/commit/3649f873a3ff7c9d01653dfd67031ab2f9548df5"><code>3649f87</code></a> Remove unused / invalid C++ lazy repeated field code from OSS. (<a href="https://redirect.github.com/protocolbuffers/protobuf/issues/19682">#19682</a>)</li>
<li><a href="https://github.com/protocolbuffers/protobuf/commit/3a7bb4a9904314cdc7c245209c94b8e975aef6f5"><code>3a7bb4a</code></a> Remove CMake downgrade workaround from Windows CI tests (<a href="https://redirect.github.com/protocolbuffers/protobuf/issues/19630">#19630</a>)</li>
<li><a href="https://github.com/protocolbuffers/protobuf/commit/d7d3da6f002a434e69e18226f24eb6a188839999"><code>d7d3da6</code></a> Merge pull request <a href="https://redirect.github.com/protocolbuffers/protobuf/issues/19506">#19506</a> from protocolbuffers/29.x-202412032207</li>
<li><a href="https://github.com/protocolbuffers/protobuf/commit/a3e9db8f0eae2d04f8298d65eac9e4d2da263d42"><code>a3e9db8</code></a> Updating version.json and repo version numbers to: 29.2-dev</li>
<li>See full diff in <a href="https://github.com/protocolbuffers/protobuf/compare/v3.29.1...v3.29.2">compare view</a></li>
</ul>
</details>
<br />

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="3.29.1" />
+    <PackageReference Include="Google.Protobuf" Version="3.29.2" />
     <PackageReference Include="Grpc.Net.Client" Version="2.65.0" />
     <PackageReference Include="Grpc.Tools" Version="2.68.1" 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="3.29.1" />
+    <PackageReference Include="Google.Protobuf" Version="3.29.2" />
     <PackageReference Include="Grpc.AspNetCore" Version="2.67.0" />
   </ItemGroup>