Add a confirmation dialog to the release script that prompts the user to manually update the release calendar documentation before proceeding with the release.

Changes

• Added a confirmation prompt that asks users to update the release calendar documentation
• Provides the URL to the documentation (https://coder.com/docs/install/releases#release-schedule)
• Suggests running the./scripts/update-release-calendar.sh script
• Requires explicit confirmation before proceeding with the release
• Exits the script if the user hasn't updated the documentation

Testing

• Script syntax validation passes (bash -n scripts/release.sh)
• Changes are placed at the appropriate point in the release flow (after release notes editing, before actual release creation)

This addresses the issue where the release calendar documentation was getting out of date. While automation can be added later, this ensures users manually confirm the documentation is updated before each release.