submodules: bump external/oasis-core from ca29d1f to 51c6778

Bumps [external/oasis-core](https://github.com/oasisprotocol/oasis-core) from `ca29d1f` to `51c6778`.
- [Release notes](https://github.com/oasisprotocol/oasis-core/releases)
- [Commits](oasisprotocol/oasis-core@ca29d1f...51c6778)

---
updated-dependencies:
- dependency-name: external/oasis-core
  dependency-version: 51c677830d9210fe64f7e0be454a2ff30610e3d1
  dependency-type: direct:production
...

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Author: dependabot[bot]