Bumps `@version` in `mix.exs` and the compiler's second carrier,
`@compiler_version` in `lib/statifier_blocks/compiler.ex`, to 0.21.0,
and the README install pin to `~> 0.21`.

Promotes the ten `changelog.d/` fragments into a `## [0.21.0]` section
dated with the local date, grouped by heading and in fragment-name
order within each, and deletes them: sb-2hoh, sb-8ykv, sb-djj5,
sb-grc1, sb-napt, sb-nyla, sb-otpv, sb-r313, sb-rd29, sb-xbyt.

Also drops the stale callback count from README's `InvokeStep`
sentence, which the pin bump has this commit touching by right
(the README half of sb-35c0; its ADR-0007 half stays open).

Refs: sb-2olh