Update LE issuer

Updates the LE issuer, should let the test pass

(Failures in https://gitlab.com/gitlab-com/content-sites/handbook-tools/handbook-monitor/-/pipelines/1817560764)

The Letsencrypt cert got renewed today

/ # hurl --test handbook.hurl
error: Assert failure
  --> handbook.hurl:6:0
   |
   | GET https://handbook.gitlab.com
   | ...
 6 | certificate "Issuer" == "C = US, O = Let's Encrypt, CN = R10"
   |   actual:   string <C = US, O = Let's Encrypt, CN = R11>
   |   expected: string <C = US, O = Let's Encrypt, CN = R10>
   |

handbook.hurl: Failure (1 request(s) in 1568 ms)
--------------------------------------------------------------------------------
Executed files:    1
Executed requests: 1 (0.6/s)
Succeeded files:   0 (0.0%)
Failed files:      1 (100.0%)
Duration:          1581 ms

/ # cat handbook.hurl
GET https://handbook.gitlab.com
HTTP 200

[Asserts]
certificate "Subject" == "CN = handbook.gitlab.com"
certificate "Issuer" == "C = US, O = Let's Encrypt, CN = R11"
certificate "Expire-Date" daysAfterNow > 15
certificate "Serial-Number" matches /[\da-f]+/

/ # hurl --test handbook.hurl
handbook.hurl: Success (1 request(s) in 1506 ms)
--------------------------------------------------------------------------------
Executed files:    1
Executed requests: 1 (0.7/s)
Succeeded files:   1 (100.0%)
Failed files:      0 (0.0%)
Duration:          1511 ms