refactor(behavior_path_planner): change variable names

[purewater0901]

Description

Change some variable names in the lane change module to improve the readability.

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[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (487c678) 12.25% compared to head (66566f2) 12.25%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3225   +/-   ##
=======================================
  Coverage   12.25%   12.25%           
=======================================
  Files        1358     1358           
  Lines       94744    94743    -1     
  Branches    26915    26915           
=======================================
  Hits        11610    11610           
+ Misses      70871    70870    -1     
  Partials    12263    12263           

Flag	Coverage Δ		*Carryforward flag
differential	5.48% <0.00%> (?)
total	12.25% <ø> (+<0.01%)	⬆️	Carriedforward from 487c678

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
...ehavior_path_planner/src/util/lane_change/util.cpp	0.00% <0.00%> (ø)

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