Update Spike2 suffixes

[CodyCBakerPhD]

From NeurodataWithoutBorders/nwb-guide#882

NeuroConv has only supported .smrx to this point; it would be a separate feature to investigate support for the .smr

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.88%. Comparing base (e2161f2) to head (dbd55b5).
Report is 131 commits behind head on main.

Files with missing lines	Patch %	Lines
...tainterfaces/ecephys/spike2/spike2datainterface.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #925   +/-   ##
=======================================
  Coverage   90.88%   90.88%           
=======================================
  Files         125      125           
  Lines        7088     7088           
=======================================
  Hits         6442     6442           
  Misses        646      646           

Flag	Coverage Δ
unittests	90.88% <50.00%> (ø)

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

Files with missing lines	Coverage Δ
...tainterfaces/ecephys/spike2/spike2datainterface.py	55.17% <50.00%> (ø)