Archive & clean up todo-java-mongo-aca template

[kristenwomack]

Archive the Azure-Samples/todo-java-mongo-aca template and remove its
references from Azure/azure-dev. Part of the todo-* template retirement epic.

Archive source repo

• Add a deprecation notice to Azure-Samples/todo-java-mongo-aca README
• Archive the Azure-Samples/todo-java-mongo-aca repository

Clean up azure-dev references

• Remove the gallery entry in cli/azd/resources/templates.json
  (repositoryPath: todo-java-mongo-aca)
• Update AI-agent sample-list recordings:
  • cli/azd/test/functional/ai_agents/testdata/recordings/Test_AIAgent_SampleList_Recorded.yaml
  • cli/azd/test/functional/ai_agents/testdata/recordings/Test_AIAgent_SampleList_JSON_Recorded.yaml
• Update DevCenter recording
  cli/azd/test/functional/testdata/recordings/Test_CLI_DevCenter_Init_Up_Down.yaml
• Sweep docs / help / pipeline comments for todo-java-mongo-aca

Historical CHANGELOG.md mentions are release history — do not edit.

Part of #9094