Fix certmagic usage. Fixes https://github.com/joohoi/acme-dns/issues/337

[maddes-b]

No separate certmagic cache needed. Default config and cache are sufficient to handle the certificates for the HTTP API.
Updated to certmagic v0.20.

Issue of double cache was introduced by 27e8251 (from #334), when updating certmagic to v0.17, and reported in #337
Also maybe related to #347, no cert, no update, no txt record.

Succesfully Tested (Ubuntu 22.04 LTS):

• normal renewal
• revoked cert renewal

Built on Ubuntu 22.04 LTS: GOPATH=/tmp/go-acme-dns CGO_ENABLED=1 go build (CGO_ENABLED for sqlite)

Installation on Ubuntu 22.04 LTS:

install -v -t /usr/local/bin/ acme-dns
install -v -d /etc/acme-dns/
install -v -t /etc/acme-dns/ config.cfg

adduser --system --gecos "acme-dns Service" --disabled-password --group --home /var/lib/acme-dns acme-dns
install -v -m u=rw-s,go=r-s -t /etc/systemd/system/ acme-dns.service
systemctl daemon-reload

systemctl enable acme-dns.service
systemctl start acme-dns.service

[maddes-b]

@joohoi This PR makes the current dev branch actually working again,
Otherwise acme-dns fails to retrieve a certificate for its own API domain.