Fix UnauthorizedAccessException during Roblox extraction

[Naiml007]

Summary

Fixes #6500.

This PR improves upgrade/extraction handling for cases where RobloxPlayerBeta.exe is temporarily unavailable and Bloxstrap throws:

System.UnauthorizedAccessException: Access to the path ... RobloxPlayerBeta.exe is denied.

Changes

• Waits for killed Roblox processes to fully exit and fails clearly if they remain running.
• Treats failure to delete the existing version directory as a real upgrade failure instead of continuing with leftover files.
• Increases retry attempts for deleting the version directory.
• Increases retry attempts for package extraction.
• Re-clears read-only attributes before extraction retries.
• Adds a clearer final error message when access is still denied, mentioning possible Roblox/antivirus/security-tool locks.

Why

The original failure can happen when Bloxstrap tries to overwrite/extract Roblox files while they are still locked by:

• a Roblox process that has not fully exited yet
• antivirus or security software scanning the executable
• read-only leftover files
• a failed cleanup of the existing version directory

These changes make transient locks more likely to recover and make persistent failures easier to diagnose.

Testing

• Ran dotnet build
• Build succeeded with 0 errors
• Existing warning only: .NET 6.0-windows is out of support