fix(modal): upgrade z-index for backdrop

[fbasso]

fix #4139

[codecov-commenter]

Codecov Report

Merging #4243 (6365dd4) into bootstrap5 (65ba6c3) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           bootstrap5    #4243   +/-   ##
===========================================
  Coverage       88.24%   88.24%           
===========================================
  Files             111      111           
  Lines            3573     3573           
  Branches          674      674           
===========================================
  Hits             3153     3153           
  Misses            372      372           
  Partials           48       48           

Flag	Coverage Δ
e2e	46.14% <ø> (ø)
unit	88.27% <ø> (ø)

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Impacted Files	Coverage Δ
src/modal/modal-backdrop.ts	100.00% <ø> (ø)

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