Send-SlackFileV2: files.upload has been deprecated

[vercellone]

Fixes #133 - Update Send-SlackFile for new Slack API

Summary

Replaces deprecated files.upload API with the new 3-step upload workflow (files.getUploadURLExternal → upload → files.completeUploadExternal) as mandated by Slack's deprecation on 11/12/2025.

Changes

• Implements new 3-step file upload flow supporting both PowerShell 7+ and legacy Windows PowerShell 3.0+
• PowerShell 7+: Uses modern -Authentication Bearer and -Token parameters
• PowerShell 3.0-5.1: Uses manual Authorization headers for compatibility
• Unified code path: Single implementation with conditional authentication based on $IsCoreCLR
• Supports both parameter sets: Content snippets and file uploads via -InFile
• Proper content types: Form-encoded for metadata, JSON for completion, octet-stream for uploads
• Maintains all existing parameters: Channel, Thread, Title, Comment, FileType, etc.
• Style consistency: Matches repo conventions (verbose logging, spacing, bracing)

Testing Status

⚠️ Untested - Based on proven PowerShell 7+ implementation but requires validation, particularly:

• Legacy PowerShell 3.0-5.1 compatibility
• File uploads (both Content and Path parameter sets)
• Thread and multi-channel targeting
• Error handling with Parse-SlackError

Breaking Changes

None - Function signature and parameters remain unchanged.

Additional Notes

• Removed legacy files.upload implementation (officially deprecated)
• Pre-signed upload URL in Step 2 requires no authentication
• Compatible with minimum PowerShell 3.0 requirement