fix(Dropdown): compute proper selectedIndex in multiple

[layershifter]

Fixes #4005.

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Introduces the same behavior as in vanilla SUI.

Before

After

[github-actions]

size-limit report

Path	Size
bundle-size/dist/Button.size.js	70.84 KB (0%)
bundle-size/dist/Icon.size.js	33.41 KB (0%)
bundle-size/dist/Image.size.js	66.13 KB (0%)
bundle-size/dist/Portal.size.js	49.9 KB (0%)

[codecov-commenter]

Codecov Report

Merging #4006 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4006   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files         184      184           
  Lines        3233     3234    +1     
=======================================
+ Hits         3228     3229    +1     
  Misses          5        5           

Impacted Files	Coverage Δ
src/modules/Dropdown/utils/getSelectedIndex.js	100.00% <100.00%> (ø)

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