Update branch references from master to main

[Copilot]

Updates all hardcoded "master" branch references to "main" in preparation for repository default branch migration.

Changes

• README.md: Updated contributing guidelines and LICENSE link URL
• ServerInstaller.cs: Changed BranchName constant to prevent installation failures

The ServerInstaller change is critical—it controls which branch git clones during Unity package installation. Without this update, installations would break when the repository switches to main as default.

// Before
private const string BranchName = "master";

// After  
private const string BranchName = "main";

Original prompt

How to make main my default branch instead of master?

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