Still a lot to do. Everywhere that `context`s are used have currently
been passed `context.TODO()`. A logger should be passed to certmagic's
lock cleanup function. Most importantly, though, it doesn't seem to be
possible to map certmagic's LibDNS providers to Lego's. In the next
commit
I will try to see how much work it is to switch to using LibDNS instead,
 which will work natively with Certmagic. The plugin registration will
 need to change, and constructors may need to be written for each
 provider. Best-case, there should be minimal changes to the
 configuration and documentation.

Updates go to 1.19. It's not the latest, but was the most compatible
version I could bump up to while requiring the least changes. quic-go
had to be updated a few versions (again, not the latest) and required
minor changes.

This PR updates all tests to work for the certmagic changes, and they
are now all passing, except for the plugin count test.

Next up is to change all of the DNS providers in tmpim/dnsproviders to
use libdns. Eventually each provider should just become minimal glue
that takes `credentials ...string`, and all the environment variables
lego supported, and returns the configured libdns provider.

A temporary Cloudflare provider has been added in
`caskettls/dnsproviders.go` to show what that would look like. The
Cloudflare provider update already has a breaking change; legacy auth
tokens are no longer supported.

The config parser now supports nesting, and all of the DNS providers
support configuration via blocks.