docs: update contribution guidelines to reference main branch

[kirisame-wang]

This pull request updates the contribution guidelines in both CONTRIBUTING.md and CONTRIBUTING-ZH.md to reference the main branch instead of the master branch.
This change aligns the documentation with the repository’s current default branch naming convention.

[gemini-code-assist]

Summary of Changes

Hello @kirisame-wang, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request ensures that the project's contribution documentation is up-to-date with the current branch naming conventions. By updating references from 'master' to 'main', new contributors will be accurately guided on where to base their work and submit pull requests, promoting consistency and clarity within the development process.

Highlights

• Documentation Update: The contribution guidelines in CONTRIBUTING.md and CONTRIBUTING-ZH.md have been updated.
• Branch Naming Convention: References to the master branch have been replaced with main to align with the repository's current default branch naming.

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[gemini-code-assist]

Code Review

This pull request correctly updates the contribution guidelines in CONTRIBUTING.md and CONTRIBUTING-ZH.md to reference the main branch instead of master. The changes are accurate and align with the repository's current branch naming convention. The update is consistent across both the English and Chinese versions of the document.