Update certificates.md

[jvanasco]

Corrected expiry year of ISRG Root X1 on Certificates page to 2035.

Important

• If this PR updates a file in content/en with a lastmod field, it must be updated.

• If this PR is a translation, please read https://github.com/letsencrypt/website/blob/master/TRANSLATION.md first.

[mcpherrinm]

This is deliberate: it is the validity in browser root stores, which is earlier than the x509 not_after

[mcpherrinm]

I think this has been a point of confusion a few times though, so some rewording could probably happen.

[jvanasco]

I updated my comment on the discourse to read the following:

The ISRG Root X1 Certificate with a "notAfter" of 2035-06-04 is considered to expire on 2030-06-04 due to root program policies. The single Certificate has a technical validity until 2035, but will expire from root programs and usage in 2030.

The confusion to me was surfaced by this thread, in which someone noted the expiry was 2035 on crt.sh and I remembered this to be 2030.

My suggestion for a fix would probably be something like this:

- until 2030-09-04 (generated 2020-09-04)
- until 2030-09-04 (notAfter: 2035-09-04; notBefore 2020-09-04)