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

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
- [x] Add a deprecation notice to `Azure-Samples/todo-java-mongo-aca` README
- [x] 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