docs(mix_generator): fold unreleased 2.1.2 notes into 2.2.0

[leoafarias]

What

mix_generator-v2.1.2 was never tagged/published (latest published tag is mix_generator-v2.1.1), so everything under its ## 2.1.2 heading is still unreleased. PR #968 bumped the package to 2.2.0 and added its own entry above the orphaned 2.1.2 section.

This folds the two unreleased 2.1.2 items up into ## 2.2.0 so they ship with this release:

• FEAT: derive nested styler types for StyleSpec<XSpec> fields by convention (from feat(mix_generator): derive nested styler types for StyleSpec fields by convention #961)
• FIX: @MixWidget generic styler call<T>() support (from fix(generator): support generic @MixWidget call methods #958)

Why this is correct

• The Breaking nested-styler change now ships under a minor bump (2.2.0) rather than the earlier patch (2.1.2) — more correct semver, and feat(mix_generator): generate variant widget constructors #968 forced the minor anyway.
• No orphan ## 2.1.2 heading left behind that would never map to a pub.dev release.

Scope

Docs-only. pubspec.yaml is already 2.2.0 (set by #968); no code, no version change. Release still launches by pushing the mix_generator-v2.2.0 tag.

[leoafarias]

Superseded: releasing as patch 2.1.2 (not 2.2.0) per maintainer decision. Packages version independently, so mix_generator ships on its own number; new release PR folds everything under 2.1.2.