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