Add deprecation warning for validate paths argument

[stephprince]

Motivation

In #1911, the validate(paths=...) argument was removed in favor of using validate(path=...) to simplify code and because validating multiple paths is a rarer use case that can be implemented by the user in a loop if needed.

To follow the typical deprecation cycle and provide a warning to users using this keyword argument, this PR readds the paths argument and issues a deprecation warning for PyNWB 4.0.

How to test the behavior?

from pynwb import validate
validate(paths=['test.nwb'])

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff check . && codespell from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.79%. Comparing base (337c914) to head (8726327).
Report is 1 commits behind head on release-3.0.0.

Additional details and impacted files

@@                Coverage Diff                @@
##           release-3.0.0    #2024      +/-   ##
=================================================
+ Coverage          92.76%   92.79%   +0.02%     
=================================================
  Files                 28       28              
  Lines               2724     2735      +11     
  Branches             712      715       +3     
=================================================
+ Hits                2527     2538      +11     
  Misses               127      127              
  Partials              70       70              

Flag	Coverage Δ
integration	73.63% <100.00%> (+0.10%)	⬆️
unit	82.63% <41.66%> (-0.19%)	⬇️

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