refactor: migrate gitlab_project_milestone to Framework

Relates to issue #6713 (closed)

Changes

This MR migrates the gitlab_project_milestone resource from Terraform Plugin SDK to Terraform Plugin Framework.

New Files Created

1. internal/provider/resource_gitlab_project_milestone.go - Framework resource implementation

   • Complete CRUD operations (Create, Read, Update, Delete)
   • State upgrade function (V0 to V1) for ID format migration from <milestone_id> to <project>:<milestone_id>
   • Import support with passthrough ID
   • Proper schema with all attributes matching SDK version
   • State management via state_event API field
   • Date parsing using gitlab.ParseISOTime()

2. internal/provider/resource_gitlab_project_milestone_test.go - Comprehensive test suite

   • TestAccGitlabProjectMilestone_StateUpgradeV0 - Unit test for state upgrade
   • TestAccGitlabProjectMilestone_basic - Full CRUD acceptance test
   • TestAccGitlabProjectMilestone_migrateFromSDKToFramework - Migration test from SDK v16.6

3. examples/resources/gitlab_project_milestone/resource.tf - Enhanced examples

   • Basic example with required fields
   • Comprehensive example showing all optional fields

Key Implementation Details

Schema (Version 1):

• project (Required, ForceNew) - Project ID or path
• title (Required, LengthAtLeast(1)) - Milestone title
• description, due_date, start_date, state (Optional, Computed)
• Computed fields: iid, milestone_id, project_id, created_at, updated_at, web_url, expired

ID Format Migration:

• V0 (SDK): <milestone_id> (single part)
• V1 (Framework): <project>:<milestone_id> (two parts)
• Automatic state upgrade for seamless migration

Special Behaviors Preserved:

• State managed via state_event API field ("activate" or "close")
• State set after creation via separate Update call
• Date parsing uses gitlab.ParseISOTime() for YYYY-MM-DD format
• All computed fields use UseStateForUnknown() plan modifiers

Testing

The implementation includes:

• ✅ Unit test for state upgrade validation
• ✅ Full CRUD lifecycle acceptance test with import verification
• ✅ Migration test validating upgrade from SDK v18.8 to Framework
• ✅ Compatibility with existing gitlab_project_milestone datasource

Next Steps

To verify the migration:

make build
make test RUN=TestAccGitlabProjectMilestone_StateUpgradeV0
make testacc-up
make testacc RUN=TestAccGitlabProjectMilestone
make reviewable
make testacc-down

The SDK resource files remain in internal/provider/sdk/ for backward compatibility during the transition period.