Harmonize softplus implementations

[ricardoV94]

There were slight differences between the implementations for different backends.

• Removed zero cutoff in C implementation. As discussed in Beta often diverges when logodds is close to 19.0 #29 (comment), this is probably not that helpful
• Added missing branch in JAX implementation.
• Clarified this Op offers both speed and stability benefits.

[codecov]

Codecov Report

Merging #884 (55acfa2) into main (14c394d) will increase coverage by 0.05%.
The diff coverage is 95.41%.

❗ Current head 55acfa2 differs from pull request most recent head cc33443. Consider uploading reports for the commit cc33443 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #884      +/-   ##
==========================================
+ Coverage   74.10%   74.15%   +0.05%     
==========================================
  Files         174      174              
  Lines       48673    48707      +34     
  Branches    10373    10372       -1     
==========================================
+ Hits        36067    36120      +53     
+ Misses      10315    10301      -14     
+ Partials     2291     2286       -5     

Impacted Files	Coverage Δ
aesara/link/jax/dispatch/scalar.py	97.40% <ø> (ø)
aesara/compile/function/types.py	79.16% <75.00%> (+0.16%)	⬆️
aesara/tensor/elemwise.py	88.07% <90.54%> (-0.52%)	⬇️
aesara/tensor/rewriting/elemwise.py	86.40% <94.44%> (+0.65%)	⬆️
aesara/scalar/basic.py	79.02% <95.16%> (+0.10%)	⬆️
aesara/compile/function/pfunc.py	84.18% <100.00%> (+1.77%)	⬆️
aesara/compile/mode.py	84.47% <100.00%> (+1.22%)	⬆️
aesara/compile/sharedvalue.py	93.75% <100.00%> (+13.26%)	⬆️
aesara/scalar/math.py	85.29% <100.00%> (+0.02%)	⬆️
aesara/scan/basic.py	84.59% <100.00%> (ø)
... and 7 more