Fix step success reporting on Gradle failure

[jhl221123]

This PR fixes an issue where GitHub Actions steps in the update-scheduled-release-version.yml workflow would incorrectly report success even if an internal Gradle task failed.

The problem was caused by wrapping the Gradle execution directly within an echo "key=$(./gradlew ...)" >> $GITHUB_OUTPUT command. The echo command itself would succeed, masking the underlying Gradle failure.

This change modifies the script to:

1. First, assign the standard output of the Gradle task to a variable.
2. Then, separately echo this variable's value to $GITHUB_OUTPUT using the key=value format.

This ensures that if the Gradle task fails, the step will now correctly report a failure.

Addresses #15
Related to spring-projects/spring-security#16764