xds: fix copy-paste bug in WeightExpirationPeriod config

[gregbarasch]

This PR fixes a copy-paste bug in the xDS client-side weighted round robin converter where GetBlackoutPeriod() was incorrectly called twice - once for BlackoutPeriod (correct) and once for WeightExpirationPeriod (incorrect).

BlackoutPeriod: The duration after startup during which weight updates are ignored
WeightExpirationPeriod: The duration after which backend weights expire if not updated

RELEASE NOTES:

• xds: Fixed incorrect proto field access in weighted round robin configuration.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: gregbarasch / name: Greg Barasch (01cb03b, dbe6368)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.19%. Comparing base (c1a9239) to head (dbe6368).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8915      +/-   ##
==========================================
+ Coverage   80.40%   83.19%   +2.79%     
==========================================
  Files         416      417       +1     
  Lines       33495    32940     -555     
==========================================
+ Hits        26930    27406     +476     
+ Misses       4682     4110     -572     
+ Partials     1883     1424     -459     

Files with missing lines	Coverage Δ
...xds/xdsclient/xdslbregistry/converter/converter.go	75.23% <100.00%> (+1.90%)	⬆️

... and 32 files with indirect coverage changes

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[arjan-bal]

LGTM, thank you for the contribution!