lib/types: add deprecation note about freeform behavior of 'either'

### Deprecations {#sec-nixpkgs-release-25.11-lib-deprecations}

- Create the first release note entry in this section!

- `types.either` silently accepted mismatching types when used in `freeformType`. Module maintainers should fix the used type

  In most cases wrapping `either` with `attrsOf` should be sufficient.

  Since types.either was used to bootstrap other types. This also affects the following types:

  - `oneOf`

  - `number`

  - `numbers.*`

### Additions and Improvements {#sec-nixpkgs-release-25.11-lib-additions-improvements}