refactor: rename gitlab_domain_name annotation to git_domain_name for non-workshop genericity

## Context

The cluster secret annotation `gitlab_domain_name` is used by ArgoCD ApplicationSet templates (in `gitops/addons/registry/`) to configure the Git server integration domain for Backstage, ArgoCD repo credentials, and other addons.

In the workshop setup, this is the EC2 GitLab CloudFront domain. In non-workshop deployments, it could be any Git server (GitHub, Bitbucket, Gitea, etc.).

## Problem

The name `gitlab_domain_name` is GitLab-specific and implies a tight coupling to GitLab. Non-workshop users may not use GitLab at all.

## Proposed Change

Rename `gitlab_domain_name` → `git_domain_name` (or `git_server_domain`) across:
- `gitops/addons/registry/*.yaml` — ApplicationSet templates
- `cluster-providers/kind-crossplane/Taskfile.yaml` — hub:seed METADATA
- `cluster-providers/kind-kro-ack/Taskfile.yaml` — hub:seed metadata
- Any helm chart templates that reference `.Values.global.gitlab_domain_name`

## Backward Compatibility

Add a fallback in templates: `{{default .metadata.annotations.git_domain_name .metadata.annotations.gitlab_domain_name}}` during transition.

## Related

- Workshop-specific usage of this field is in `cluster-providers/workshop/Taskfile.yaml` (reads from `private/gitlab-cloudfront-domain`)
- This is a non-breaking rename if done with the fallback pattern

Start by searching for gitlab_domain_name across gitops/addons/registry/, both listed cluster-provider Taskfiles, and Helm chart templates. Review the workshop Taskfile reference and inspect the rendered ApplicationSet and chart values to confirm the new generic annotation and transition fallback work. Done means all specified references use the generic name while the legacy annotation remains supported.

Written by the indexing model from the issue text.