Bump github.com/mholt/certmagic from f00a66c to 707b204

Bumps [github.com/mholt/certmagic](https://github.com/mholt/certmagic) from `f00a66c` to `707b204`.
- [Release notes](https://github.com/mholt/certmagic/releases)
- [Commits](caddyserver/certmagic@f00a66c...707b204)

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