Ensure DiscreteSlider label is updated

[philippjfr]

Fixes #3275 and #2709

[codecov]

Codecov Report

Merging #3278 (203250d) into master (2218cdc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3278   +/-   ##
=======================================
  Coverage   83.28%   83.29%           
=======================================
  Files         195      195           
  Lines       26156    26166   +10     
=======================================
+ Hits        21785    21794    +9     
- Misses       4371     4372    +1     

Impacted Files	Coverage Δ
panel/tests/io/test_embed.py	99.41% <100.00%> (+0.01%)	⬆️
panel/widgets/slider.py	84.12% <100.00%> (-0.04%)	⬇️
panel/tests/pane/test_plotly.py	98.15% <0.00%> (-0.62%)	⬇️

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