chore: bump iii SDK pins to 0.21.6

[iii-hq-ci]

What changed

• bump all iii template versions and minimum engine versions to 0.21.6
• bump iii-sdk, iii-helpers, and @iii-dev/helpers pins to 0.21.6
• synchronize the Node and Rust lockfiles in the multi-worker orchestration template

Why

iii 0.21.6 includes managed worker-name propagation into worker VMs, with the SDKs using that configured name by default. Keeping the templates and their lockfiles on the matching SDK release ensures iii worker add --wait, iii worker status, and iii worker list identify scaffolded workers consistently.

Impact

Newly scaffolded projects and workers use the 0.21.6 engine/SDK contract. Existing projects are unchanged.

Validation

• Init Smoke: main channel passed
• Init Smoke: next channel passed
• python scripts/bump_versions.py --version 0.21.6 reports no remaining manifest changes
• npm ci --ignore-scripts passed for the client and payment workers
• cargo check --locked passed for the compute worker
• the quickstart math and caller workers registered as 0.21.6, and math::add_two_numbers returned c=30