Bump getgrav/grav from 1.7.25 to 1.7.28 (#14)

Bumps [getgrav/grav](https://github.com/getgrav/grav) from 1.7.25 to 1.7.28.
- [Release notes](https://github.com/getgrav/grav/releases)
- [Changelog](https://github.com/getgrav/grav/blob/develop/CHANGELOG.md)
- [Commits](getgrav/grav@1.7.25...1.7.28)

---
updated-dependencies:
- dependency-name: getgrav/grav
  dependency-type: indirect
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>