Bump src/external/gsl from 87f9d76 to ddae9d7

Bumps [src/external/gsl](https://github.com/microsoft/GSL) from `87f9d76` to `ddae9d7`.
- [Release notes](https://github.com/microsoft/GSL/releases)
- [Commits](microsoft/GSL@87f9d76...ddae9d7)

---
updated-dependencies:
- dependency-name: src/external/gsl
  dependency-type: direct:production
...

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