Bump web-ext in /glean/tests/unit/platform/utils/webext/sample

Bumps [web-ext](https://github.com/mozilla/web-ext) from 6.1.0 to 6.3.0.
- [Release notes](https://github.com/mozilla/web-ext/releases)
- [Commits](mozilla/web-ext@6.1.0...6.3.0)

---
updated-dependencies:
- dependency-name: web-ext
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]