Fix pre-post release workflow: missing inputs field

[JonasKunz]

Trying to trigger the pre-release job yielded the following error:

[Invalid workflow file: .github/workflows/pre-release.yml#L21](https://github.com/elastic/ecs-logging-java/actions/runs/7902626610/workflow)
The workflow is not valid. In .github/workflows/pre-release.yml (Line: 21, Col: 11): Error from called workflow elastic/ecs-logging-java/.github/workflows/pre-post-release.yml@43a916f13657d1c9941332340219b447347e5c4e (Line: 6, Col: 5): Unexpected value 'ref' In .github/workflows/pre-release.yml (Line: 21, Col: 11): Error from called workflow elastic/ecs-logging-java/.github/workflows/pre-post-release.yml@43a916f13657d1c9941332340219b447347e5c4e (Line: 10, Col: 5): Unexpected value 'version'

Based on my reading of the docs the input variables should be nested under an inputs field