Bump ol from 6.6.1 to 6.8.1

Bumps [ol](https://github.com/openlayers/openlayers) from 6.6.1 to 6.8.1.
- [Release notes](https://github.com/openlayers/openlayers/releases)
- [Commits](openlayers/openlayers@v6.6.1...v6.8.1)

---
updated-dependencies:
- dependency-name: ol
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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