* release: svy 0.21.0, svy-rs 0.12.0

svy-rs clears the 0.12 boundary rather than taking a patch. svy 0.20.1
pins svy-rs>=0.11.0,<0.12.0, so a 0.11.1 would reach every existing
install on the next sync and silently change domain df, by= confidence
intervals and deff, with no svy upgrade to account for it. A minor bump
keeps the behaviour change behind an explicit svy upgrade.

svy takes a minor rather than a patch because Estimate.degrees_freedom
is removed from the public API and EstimateData.degrees_freedom from
the serialized payload (SCHEMA_VERSION svy-result/0.2).

Both svy-rs version strings move together: Cargo.toml for the crate and
pyproject.toml for the Python distribution. maturin reads the latter,
so bumping only Cargo.toml leaves the version the pin resolves against
untouched.

* docs(changelog): promote Unreleased to svy 0.21.0 and svy-rs 0.12.0