Enable http/2 support

- As per [the documentation](https://pkg.go.dev/net/http#Serve), it
doesn't enable HTTP2 by-default, unless we enable it via the
`NextProtos` option.

server/certificates/certificates.go

@@ -117,6 +117,7 @@ func TLSConfig(mainDomainSuffix string,
 		},
 		PreferServerCipherSuites: true,
 		NextProtos: []string{
+			"h2",
 			"http/1.1",
 			tlsalpn01.ACMETLS1Protocol,
 		},