[release/3.1] (deps): Bump src/submodules/MessagePack-CSharp

Bumps [src/submodules/MessagePack-CSharp](https://github.com/aspnet/MessagePack-CSharp) from `8861abd` to `6caf299`.
- [Release notes](https://github.com/aspnet/MessagePack-CSharp/releases)
- [Commits](aspnet/MessagePack-CSharp@8861abd...6caf299)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]