more flexibility in keeping "sloppy" parts of existing license header

If you are submitting a **bug**, please include the following:

- [x] summary of problem
- [x] gradle or maven version
- [x] spotless version
- [ ] operating system and version
- [ ] copy-paste your full Spotless configuration block(s), and a link to a public git repo that reproduces the problem if possible
- [ ] copy-paste the full content of any console errors emitted by `gradlew spotless[Apply/Check] --stacktrace`

If you're just submitting a feature request or question, no need for the above.

* Spotless 6.16.0
* Gradle 8.0.1
* MacOS Ventura
```

spotless {
java {
licenseHeader 'SPDX-License-Identifier: Apache-2.0'
ignoreErrorForStep('LicenseHeaderStep')
}
}
```

1. Regex

Our large codebase has some variations in copyright specifically in terms of spaces between comment chars & the license text

So our `checkstyle` config (not merged, just trying to find a good tool to add this check since we migrated from maven) becomes
```

```

That seems good enough, so I'm wondering if I can use spotless instead, as it offers more functionality which we may use in future

Obviously we have have a preferred spacing (for auto fix) but want to tolerate additional spaces

I tried without the comments, but then all files fail the check

Interestingly I also wanted to only report warnings, and note that though multiple errors on a project will now be reported, in a multi module build, the build will terminate as that project build fails (unless --continue is used).

Finally, in the example above I'd need two license header steps, but this seems to not be allowed?