Default labels conflict with Dependabot

Some of the default labels created by `auto` — namely, "major", "minor", and "patch" — happen to have special meaning to [Dependabot](https://docs.github.com/en/code-security/dependabot). Whenever Dependabot creates a pull request for a major version update (e.g., updating a dependency from v3 to v4), if the repository the PR is created in has a "major" label defined, Dependabot will apply the "major" label to that PR (and likewise for "minor" and "patch"), which will affect the version bump level calculated by `auto`, likely in a way that the user does not want. [There is currently no way to stop Dependabot from doing this](https://github.com/dependabot/dependabot-core/issues/3465), so the next best thing would be for `auto` to change its default labels to something that doesn't have meaning to Dependabot. For example, for some of our projects, we use "semver-major", "semver-minor", and "semver-patch".