submodules: bump external/oasis-core from 914f7c4 to 6987cde

Bumps [external/oasis-core](https://github.com/oasisprotocol/oasis-core) from `914f7c4` to `6987cde`.
- [Release notes](https://github.com/oasisprotocol/oasis-core/releases)
- [Commits](oasisprotocol/oasis-core@914f7c4...6987cde)

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

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