Sanitize the color field to allow using hex codes

[pjpires]

Found myself struggling with the color field, as I was unfamiliar with the underlying GitHub API.

This PR fixes #153 and adds a test scenario for hex code colors.
Replaces and closes #172.

[crazy-max]

Looks good but can you:

• squash your commits
• run docker buildx bake pre-checkin per https://github.com/crazy-max/ghaction-github-labeler/blob/master/.github/CONTRIBUTING.md#submitting-a-pull-request
• push commit with generated files changes similar to https://github.com/crazy-max/ghaction-github-labeler/pull/187/commits with commit message update generated content

Thanks!

[pjpires]

Looks good but can you:

* squash your commits

* run `docker buildx bake pre-checkin` per [`master`/.github/CONTRIBUTING.md#submitting-a-pull-request](https://github.com/crazy-max/ghaction-github-labeler/blob/master/.github/CONTRIBUTING.md?rgh-link-date=2024-02-06T14%3A26%3A35Z#submitting-a-pull-request)

* push commit with generated files changes similar to [#187 (commits)](https://github.com/crazy-max/ghaction-github-labeler/pull/187/commits) with commit message `update generated content`

Thanks!

Sorry, I've read the contributing instructions but I didn't get that you meant the result of running docker buildx bake pre-checkin to be a separate commit. Can you please double check if everything looks good now? Thanks!

[crazy-max]

LGTM thanks!