Refactor prod_ops_slackbot script to work with relative dates
Overview
Beginning with the 16.6 milestone, we are moving to a 3rd Thursday release schedule instead of the usual 22nd of the month.
There are a number of process and automation dependencies that are requires to changes as a result. One of those required changes are the automated reminder messages posted to Slack.
Note: With Temporarily disables release post slackbot anno... (!838 - merged) we are temporarily disabling these announcements until the relative date behavior is introduced to avoid confusion.
Acceptance Criteria
- use the milestone release dates found in https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/ec78bedc19f7d0a4cbfc06efc061d496d7013aee/data/releases.yml, update the behavior of the prod_ops_slackbot script to use dates relative to the release date.
- update the configuration to fit with the relative dates to reenable this behavior
Notes
- In other automation for building the release post, this is how we've approached that offset math.