Bump github.com/mholt/certmagic from c1d472b to f92e853

Bumps [github.com/mholt/certmagic](https://github.com/mholt/certmagic) from `c1d472b` to `f92e853`.
- [Release notes](https://github.com/mholt/certmagic/releases)
- [Commits](caddyserver/certmagic@c1d472b...f92e853)

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