Bump src/external/gsl from 4300304 to 52212c2 (#53)

Bumps [src/external/gsl](https://github.com/microsoft/GSL) from `4300304` to `52212c2`.
- [Release notes](https://github.com/microsoft/GSL/releases)
- [Commits](microsoft/GSL@4300304...52212c2)

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

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