Remove update-gradle-wrapper action (#169)

Dependabot now supports updating the Gradle wrapper directly via the
existing `gradle` ecosystem in `.github/dependabot.yml`, making the
dedicated `gradle-update/update-gradle-wrapper-action` workflow
redundant — it was producing duplicate PRs alongside Dependabot's.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ardetrick