Bump src/external/gsl from 756c91a to 0249144 (#92)

Bumps [src/external/gsl](https://github.com/microsoft/GSL) from `756c91a` to `0249144`.
- [Release notes](https://github.com/microsoft/GSL/releases)
- [Commits](microsoft/GSL@756c91a...0249144)

---
updated-dependencies:
- dependency-name: src/external/gsl
  dependency-version: '0249144ad1487bea10a2b553918e581c0c16a1c5'
  dependency-type: direct:production
...

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