docs: replace deprecated PingCAP domains

[wuhuizuo]

Summary

• replace deprecated download.pingcap.org references with download.pingcap.com
• replace deprecated charts.pingcap.org references with charts.pingcap.com
• keep existing paths and protocols unchanged

Validation

• git diff --check
• verified no remaining download.pingcap.org / charts.pingcap.org references in the patched branch

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[gemini-code-assist]

Code Review

This pull request updates the download URL in scripts/upload.py from pingcap.org to pingcap.com. I have no further feedback to provide.