Let's Encrypt planned the transition to ISRG's root certificate ("ISRG Root
X1") on July 8, 2019, but postponed several times.
Finally they found another solution: A certificate 'ISRG Root X1', but
cross-signed with 'DST Root CA X3' and with a livetime that exceeds that
of the root CA. This is said to work for most operating system where root
certificate authorities are just 'trust anchors'.
I doubt this is true for RouterOS, where certificates are just imported
into the certificate store. So let's migrate to 'ISRG Root X1' now.
Also order certificates, so we have:
* intermediate
* root
* alternative root, if any
Let's add 'ISRG Root X1' for 'E1' as there will be a valid cross-signed
chain 'E1' -> 'ISRG Root X2' -> 'ISRG Root X1'.
