Create a GitLab release calendar

Context

gitlab-com/gl-infra/software-delivery&1 (closed) proposes to move the active stable branch generation one week earlier in the release schedule to allow for thorough testing and timely backports. This will increase the quality, safety, and reliability of the monthly release while enabling a better developer experience and confidence.

Problem

Currently, it is hard for engineers to figure out if a code change was included in the monthly release. The release dashboard offers a visualization of the upcoming release dates, however, it can be misleading:

• It shows the release dates, instead of a date to guarantee inclusion (to be fixed by #21096 (closed))
• It only shows the dates for the upcoming monthly and patch release, not the dates for subsequent ones.

Proposal: Create a release calendar

A release calendar could be established in the handbook, indicating:

• A date is specified for each release that guarantees feature inclusion ("code complete" / "feature complete" date), to allow stage teams proper planning. Likely, the initial release candidate date
• A release date for each patch release, including a due date to include bug and security fixes.

Prototype: