[atlas] Letsencrypt certificates on anchors

Dear colleagues, Some time ago, there was discussion on this list about deploying Letsencrypt certificates on anchors. We had said we would investigate the possibility of doing this. We are pleased to report that we have worked out how to do this seamlessly. Early next week, all RIPE Atlas anchors will switch to Letsencrypt certificates. Note that we will re-use the existing keys on the anchors to request the Letsencrypt certificates. The TLSA records of all anchors are pinned to these private keys, and so they will not change. Regards, Anand Buddhdev RIPE NCC