config-conventional: Remove support for the "improvement" type

[gavinsharp]

Expected Behavior

Commit messages that use the "improvement" type should be rejected when using the @commitlint/config-conventional config.

• See Add improvement to allowed list. #832 (comment) for rationale.
  • Short version is that it was only ever added in a beta version of the spec, and most other tools in the ecosystem have not added support for it.

Current Behavior

"improvement" was allowed in #832

Affected packages

• config-conventional

Context

This caused unexpected behaviour in our app because commitlint allowed the commit, but semantic-release does not recognize that commit type, so it did not generate a release.

commitizen also reverted their addition of this type: https://github.com/commitizen/conventional-commit-types/pull/16/files