Extend gitlab-agent release process to GitLab Agent Helm Chart

With KAS (part of gitlab-agent) under Managed Versioning after Introduces KAS release process (gitlab-org/release-tools!2860 - merged) and with Run gitlab-agent pipeline on releases to deploy... (#20168 - closed) fixed we should be ready to extend the gitlab-agent release process to creating releases for the GitLab Agent Helm Chart.

To release the GitLab Agent Helm Chart we need to update the appVersion to match the gitlab-agent (KAS and agentk - it's the same version) release version and to update the version (incrementing minor or rarely, major - let's focus on minors) in the Chart settings and tag it so that the release pipeline runs.

This doesn't have to address security releases for the Helm Chart in an initial iteration. I think just releasing it in the normal scenario already helps to avoid manual labor.