Dropping support for Bokeh 2

[hoxbro]

#5751 removed Bokeh 2 from the test suite.

This PR continues it and removes support from the code base.

The sorting of import will be done in another PR, where I will enable automatic linting for it.

[codecov-commenter]

Codecov Report

Merging #5891 (d2fd3e4) into main (3c414fb) will increase coverage by 0.15%.
Report is 14 commits behind head on main.
The diff coverage is 96.36%.

@@            Coverage Diff             @@
##             main    #5891      +/-   ##
==========================================
+ Coverage   88.41%   88.56%   +0.15%     
==========================================
  Files         312      313       +1     
  Lines       64953    65069     +116     
==========================================
+ Hits        57427    57628     +201     
+ Misses       7526     7441      -85     

Flag	Coverage Δ
ui-tests	23.28% <34.54%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
holoviews/plotting/bokeh/callbacks.py	62.20% <100.00%> (-0.12%)	⬇️
holoviews/plotting/bokeh/element.py	88.14% <100.00%> (+0.47%)	⬆️
holoviews/plotting/bokeh/graphs.py	91.25% <ø> (+0.21%)	⬆️
holoviews/plotting/bokeh/links.py	72.35% <100.00%> (+1.56%)	⬆️
holoviews/plotting/bokeh/plot.py	93.13% <100.00%> (+1.28%)	⬆️
holoviews/plotting/bokeh/raster.py	85.09% <100.00%> (+2.04%)	⬆️
holoviews/tests/plotting/bokeh/test_elementplot.py	96.91% <ø> (+0.10%)	⬆️
holoviews/tests/plotting/bokeh/test_graphplot.py	100.00% <100.00%> (+0.75%)	⬆️
holoviews/tests/plotting/bokeh/test_gridplot.py	100.00% <100.00%> (+2.19%)	⬆️
holoviews/tests/plotting/bokeh/test_layoutplot.py	100.00% <100.00%> (+9.44%)	⬆️
... and 4 more

... and 35 files with indirect coverage changes

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[philippjfr]

Great, thank you! All the changes seem straightforward and uncontroversial. LGTM

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.