Add .java-version file

[ccjernigan]

Is your feature request related to a problem? Please describe.

The Java version used for builds is spread across several files, including GitHub Actions workflows and Gradle scripts.

Describe the solution you'd like

Add a .java-version file and remove the version in the GitHub Actions scripts. This provides a single version for all the actions scripts.

Then modify the check properties Gradle task to ensure .java-version is in sync with the version declared in the Gradle scripts.

After this change, java version will only be declared in two places and it will be self-checking on CI to confirm the two versions are in sync.