Have consistent date format for project releases

This change improves how the GitLab release resource handles release dates by switching from basic string handling to a specialized time type that properly validates RFC3339 date formats (like "2019-03-15T08:00:00Z").

The main improvements include:

• Better error handling when users provide invalid date formats - the system now catches malformed dates earlier and gives clearer error messages
• More robust date parsing that follows standard RFC3339 format requirements
• Added comprehensive tests to verify the system correctly handles future dates, past dates, and rejects invalid date formats
• Simplified the internal code by removing custom date parsing logic in favor of the framework's built-in time handling

This makes the release date feature more reliable and user-friendly by catching date format errors upfront rather than failing later in the process.

Fixes #6752 (closed)