Migrate to official go-sdk for MCP Server

[ibakshay]

Currently, the GitHub MCP server relies on mark3labs/mcp-go, a community-maintained implementation of the Model Context Protocol.

With the release of the official MCP SDK for Go by the Model Context Protocol team, we should migrate our implementation to leverage the official library. This ensures better long-term alignment with the protocol specification, improved consistency, and access to official updates and support.