build(deps): bump github.com/sigstore/fulcio from 1.6.3 to 1.8.3

Bumps [github.com/sigstore/fulcio](https://github.com/sigstore/fulcio) from 1.6.3 to 1.8.3.
- [Release notes](https://github.com/sigstore/fulcio/releases)
- [Changelog](https://github.com/sigstore/fulcio/blob/main/CHANGELOG.md)
- [Commits](sigstore/fulcio@v1.6.3...v1.8.3)

---
updated-dependencies:
- dependency-name: github.com/sigstore/fulcio
  dependency-version: 1.8.3
  dependency-type: direct:production
...

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Author: dependabot[bot]