Run gitlab-agent pipeline on releases to deploy agentk

We've recently switched to Managed Versioning for ServiceKAS.

See Introduces KAS release process (gitlab-org/release-tools!2860 - merged)

The issue is that we still build agentk in the canonical gitlab-agent repository (see my comment in Define security release process for GitLab Agent for Kubernetes), even though we build KAS via CNG (they are in the same repository).

We've been using the release pipeline (run on version tags) in the canonical repository to build and deploy the agentk binaries / images and also create the GitLab Release for gitlab-agent. (for example, currently the renovate integration is broken, because there wasn't a GitLab Release for v16.11.0).

We should fix this asap as currently we don't have any v16.11.0 release assets for agentk.

See https://gitlab.slack.com/archives/C015U1TKV4M/p1713695886307589