[cabf_validation] Clarifying Acceptable Status Codes for Following Redirects in methods 18 and 19

[Jacob Hoffman-Andrews]

At Let's Encrypt, we use Go's net/http for processing redirects:
https://godoc.org/net/http#Client.Do. That means we currently follow
301, 302, 303, 307, and 308. The library does have the flexibility to
specifically exclude 303, so we can make that change if needed. I
don't think it's common for our subscribers to use it.