Introduce legacy namespace fields for managed resources

What does this MR do and why?

Introduce legacy namespace fields for managed resources

This change set introduces a new legacy namespace field for GitLab-managed resources templates.

Related work has been implemented in KAS with:

• gitlab-org/cluster-integration/gitlab-agent!2986 (merged)
• gitlab-org/cluster-integration/gitlab-agent!2987 (merged)

Refs gitlab-org/cluster-integration/gitlab-agent#744 (closed)

Validation

• Setup KAS with an agent and GitLab-managed resources.
• Run such a pipeline
• Verify an object in the cluster to verify the annotations that look something like this:

# Please edit the object below. Lines beginning with a '#' will be ignored,
# and an empty file will abort the edit. If an error occurs while saving this file will be
# reopened with the relevant failures.
#
apiVersion: v1
kind: Namespace
metadata:
  annotations:
    agent.gitlab.com/env-manres2-20-1: '{"environment_id":107,"environment_name":"manres2","environment_slug":"manres2","environment_page_url":"https://gdk.test:3443/root/agentk-test/-/environments/107","environment_tier":"other","agent_id":1,"agent_name":"k3d-agentk","agent_url":"https://gdk.test:3443/root/agentk-test/-/cluster_agents/k3d-agentk","project_id":20,"project_slug":"agentk-test","project_path":"root/agentk-test","project_url":"https://gdk.test:3443/root/agentk-test","template_name":"default","project_rfc1123_slug":"agentk-test","legacy_namespace":"agentk-test-20manres2"}'
  creationTimestamp: "2025-09-25T11:09:02Z"
  labels:
    agent.gitlab.com/env-manres2-20-1: ""
    agent.gitlab.com/environment_slug-manres2: ""
    agent.gitlab.com/id-1: ""
    agent.gitlab.com/project_id-20: ""
    kubernetes.io/metadata.name: manres2-20-1
  name: manres2-20-1
  resourceVersion: "3722661"
  uid: e7b358ec-4a65-4a38-b4b4-5dbe64676345
spec:
  finalizers:
  - kubernetes
status:
  phase: Active

Note the new legacy_namespace1 field in the agent.gitlab.com/env-*** annotation.

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