Linter: ensure feedstock-maintainers are part of conda-forge

Some CODEOWNERS files are buggy because we assume feedstock maintainers have accepted the invitation once the feedstock is created after merge in staged-recipes, but that's not always the case. Example: https://github.com/conda-forge/reskit-feedstock/blob/02420157ea7e05f7e43e151ea7ad559256584fed/.github/CODEOWNERS

Should we lint/hint that?

