Retired Certificates (#1159)
Retired Certificates (#1159)

* Retired Certificates

Moved "Let's Encrypt Authority X3" and "Let's Encrypt Authority X4" to the retired section and updated corresponding text to reflect these retirements and the transition to R3. Added overlooked crt.sh link to "ISRG Root OCSP X1".

* Update certificates.md

fixed lastmod
letsencrypt/website,letsencrypt/website,letsencrypt/website