Extract Logic for Computing Versions into a Separate GitHub Action

[jzheaux]

It would be nice to be able to do:

- uses: spring-io/version-compute-action

This action would look at the gradle.properties or pom.xml file to determine the current version of the project.

See also https://github.com/spring-io/spring-github-workflows/blob/a2a11ad54cdbe8890741a37359dbcb581c866444/.github/actions/spring-scheduled-milestone-for-branch/action.yml#L31