remove deprecated slicing semantics of polynomials

[fchapoton]

📚 Description

This removes the deprecations introduced in #18940. So the only allowed slicing of polynomials is p[:k].

📝 Checklist

• I have linked an issue or discussion.
• I have made sure that the title is self-explanatory and the description concisely explains the PR.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[codecov-commenter]

Codecov Report

Patch coverage: 98.01% and project coverage change: +0.01 🎉

Comparison is base (52a81cb) 88.57% compared to head (75e3f31) 88.59%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #35205      +/-   ##
===========================================
+ Coverage    88.57%   88.59%   +0.01%     
===========================================
  Files         2140     2140              
  Lines       397273   397406     +133     
===========================================
+ Hits        351891   352081     +190     
+ Misses       45382    45325      -57     

Impacted Files	Coverage Δ
src/sage/schemes/elliptic_curves/ell_generic.py	93.25% <66.66%> (+0.01%)	⬆️
src/sage/interfaces/tachyon.py	87.93% <90.00%> (+0.43%)	⬆️
src/sage/schemes/elliptic_curves/gal_reps.py	82.23% <90.00%> (+0.04%)	⬆️
src/sage/quadratic_forms/quadratic_form.py	90.26% <95.65%> (+0.16%)	⬆️
src/sage/modular/quasimodform/element.py	99.20% <100.00%> (+0.06%)	⬆️
...l/padics/polynomial_padic_capped_relative_dense.py	67.91% <100.00%> (-0.06%)	⬇️
...age/rings/polynomial/polynomial_element_generic.py	92.14% <100.00%> (+0.12%)	⬆️
src/sage/rings/qqbar.py	95.30% <100.00%> (+<0.01%)	⬆️
src/sage/schemes/affine/affine_morphism.py	90.33% <100.00%> (ø)
src/sage/schemes/elliptic_curves/BSD.py	43.75% <100.00%> (+0.21%)	⬆️
... and 77 more

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[fchapoton]

ok, changed the error to IndexError

[mkoeppe]

One doctest output needs updating - https://github.com/sagemath/sage/actions/runs/4347013220/jobs/7593799402#step:11:7798

[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: 75e3f31