build(deps): bump semver in /plugins/gatsby-plugin-image-generator (#47)

Bumps [semver](https://github.com/npm/node-semver) from 7.3.7 to 7.7.3.
- [Release notes](https://github.com/npm/node-semver/releases)
- [Changelog](https://github.com/npm/node-semver/blob/main/CHANGELOG.md)
- [Commits](npm/node-semver@v7.3.7...v7.7.3)

---
updated-dependencies:
- dependency-name: semver
  dependency-version: 7.7.3
  dependency-type: indirect
...

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Author: dependabot[bot]