ci: replace deprecated PingCAP domains

[wuhuizuo]

Summary

• replace deprecated PingCAP download/charts domain references in the diag release pipeline
• keep existing paths and protocols unchanged

Validation

• git diff --check
• verified no remaining old-domain matches in the touched file

[pingcap-cla-assistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 8.46%. Comparing base (82f9593) to head (b611ee9).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #495      +/-   ##
=========================================
- Coverage   10.70%   8.46%   -2.23%     
=========================================
  Files         124     124              
  Lines       12591   10445    -2146     
=========================================
- Hits         1347     884     -463     
+ Misses      11073    9406    -1667     
+ Partials      171     155      -16     

Flag	Coverage Δ
unittests	8.46% <ø> (-2.23%)	⬇️

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