[release/9.0] (deps): Bump src/submodules/MessagePack-CSharp (#58183)

Bumps [src/submodules/MessagePack-CSharp](https://github.com/aspnet/MessagePack-CSharp) from `ecc4e18` to `9511905`.
- [Commits](aspnet/MessagePack-CSharp@ecc4e18...9511905)

---
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]