Drop pynwb < 3.1 compatibility code from pynwb_utils

[CodyCBakerPhD]

Yet another split out of #1933 to clean up some dead code

The declared floor in the pyproject.toml is pynwb >= 3.1.0 (and nwbinspector >= 0.7.0 requires pynwb >= 3.1 anyway), which makes several version guards unreachable:

• validate(): only the pynwb.validate(path=...) branch (pynwb >= 3.0) can run; the paths=[...] tuple-returning form (2.2 to 2.x) and the io= fallback for even older releases are gone.
• copy_nwb_file(): cache_spec has been accepted by export() since pynwb 2.8.2, so pass it unconditionally.
• _get_external_images(): ExternalImage exists in every supported pynwb, so import it at module level instead of guarding an ImportError.

The now-unused packaging.version.Version import is removed as well. The NWB schema version checks (e.g. the < 2.1.0 error filter) are about the file being validated, not about pynwb, and are unchanged.

Claude-Session: https://claude.ai/code/session_01HFhMPs6vF5zyeHDvPTvRcA

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.35%. Comparing base (4f316f7) to head (023745f).

Files with missing lines	Patch %	Lines
dandi/pynwb_utils.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                           @@
##           claude/validate-broken-symlink-path    #1939      +/-   ##
=======================================================================
+ Coverage                                78.33%   78.35%   +0.02%     
=======================================================================
  Files                                       92       92              
  Lines                                    14087    14075      -12     
=======================================================================
- Hits                                     11035    11029       -6     
+ Misses                                    3052     3046       -6     

Flag	Coverage Δ
unittests	78.35% <50.00%> (+0.02%)	⬆️

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[CodyCBakerPhD]

Note: code is technically covered by the py3-lowest tox environment test that doesn't upload to codecov (could fix that after this stack, seems minor)