docs: add Go third-party notices

[dillon-zheng]

Summary

Add a self-contained root ThirdPartyNotices.txt for the repository's Go dependencies.

Scope

• Linux/amd64 go list -deps ./..., with tests excluded.
• Excludes the Go standard library and first-party github.com/pingcap/ and github.com/tikv/ modules.
• Generated from source commit 1906ee1cb5494fadab9314392a37b50b6371f1f8 using us-docker.pkg.dev/pingcap-testing-account/internal/test/notice-generator@sha256:60fd803ae1ff5d74cc158cc9190d27c2b0bef4b892bcbe8d9b7de0c1ae2252d5 (Go 1.25.12, go-licenses v1.6.0).

Validation

• Strict collector check passed for 87 components.
• Verified the rendered file embeds license texts and required upstream notices, and contains no generator-local paths or raw collector URLs.
• git diff --check

No application code changed.

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