Update mutating step list in --gremlin-filters

[Cole-Greer]

Issue #, if available:

Description of changes:

Updates --gremlin-filters to block use of mutating steps mergeV() and mergeE()

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (79a2278) 44.84% compared to head (e6dd439) 44.81%.
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #102      +/-   ##
===========================================
- Coverage    44.84%   44.81%   -0.03%     
===========================================
  Files          247      247              
  Lines         8550     8550              
  Branches       881      881              
===========================================
- Hits          3834     3832       -2     
- Misses        4420     4423       +3     
+ Partials       296      295       -1     

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