Fix RangeSlider value_start/value_end readonly

[philippjfr]

Thanks @jbednar for catching this.

[codecov]

Codecov Report

Merging #2468 (217c35d) into master (668fad3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2468   +/-   ##
=======================================
  Coverage   83.28%   83.28%           
=======================================
  Files         184      184           
  Lines       23081    23082    +1     
=======================================
+ Hits        19222    19223    +1     
  Misses       3859     3859           

Impacted Files	Coverage Δ
panel/widgets/slider.py	76.00% <100.00%> (+0.05%)	⬆️

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