Update devcontainer Docker-in-Docker feature references to v3

[PabloZaiden]

Summary

• Updated devcontainer templates and example workspaces to use Docker-in-Docker feature v3.
• Adjusted template-related tests to expect the v3 feature reference.

[Copilot]

Pull request overview

This PR updates the repository’s devcontainer templates and example workspace configs to reference the Docker-in-Docker devcontainer feature v3, and aligns the test suite expectations with that new reference.

Changes:

• Bumped ghcr.io/devcontainers/features/docker-in-docker from :2 to :3 in template generation (src/templates.ts).
• Updated example devcontainer configs to use the v3 feature reference.
• Updated tests to expect the v3 feature reference in generated configs and pinned reference strings.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/templates.ts	Updates the shared Docker-in-Docker feature constant to v3 so all built-in templates use it.
tests/templates.test.ts	Adjusts template assertions/pinnedReference expectations to match the v3 feature reference.
tests/examples.test.ts	Updates expected generated devcontainer config feature refs and pinnedReference to v3.
tests/core.test.ts	Updates resolveWorkspaceConfig test expectation to v3.
examples/smoke-workspace/.devcontainer/devcontainer.json	Updates example workspace devcontainer feature ref to v3.
examples/complex-workspace/.devcontainer/devcontainer.json	Updates example workspace devcontainer feature ref to v3.
.devcontainer/devcontainer.json	Updates the repo’s devcontainer feature ref to v3.

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