Bump proj4 from 2.20.4 to 2.20.6

Bumps [proj4](https://github.com/proj4js/proj4js) from 2.20.4 to 2.20.6.
- [Release notes](https://github.com/proj4js/proj4js/releases)
- [Changelog](https://github.com/proj4js/proj4js/blob/master/changelog.md)
- [Commits](proj4js/proj4js@v2.20.4...v2.20.6)

---
updated-dependencies:
- dependency-name: proj4
  dependency-version: 2.20.6
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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Author: dependabot[bot]