ci: fix deployment notifier reaching slack block size
Created by: jhchabran
Slack has a fixed limit of 3000 chars per block, and due to the currently low frequency of preprod updates, it means we're getting a lot of PRs.
This expedited PR fixes it by breaking the paragraph in separate blocks.
Test plan
Tested locally: https://sourcegraph.slack.com/archives/C02L31KTHHT/p1648228774060979