[release/6.0] (deps): Bump src/submodules/MessagePack-CSharp (#58744)

Bumps [src/submodules/MessagePack-CSharp](https://github.com/aspnet/MessagePack-CSharp) from `9511905` to `9aeb12b`.
- [Commits](aspnet/MessagePack-CSharp@9511905...9aeb12b)

---
updated-dependencies:
- dependency-name: src/submodules/MessagePack-CSharp
  dependency-type: direct:production
...

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

Author: dependabot[bot]