Update FluxModelApproximator references to FluxApproximator

[jeremiahpslewis]

PR Checklist

• Update NEWS.md?
• Unit tests for all structs / functions?
• Integration and correctness tests using a simple env?
• PR Review?
• Add or update documentation?
• Write docstrings for new methods?

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 57.57%. Comparing base (bf37d4d) to head (fef5e44).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1051      +/-   ##
==========================================
+ Coverage   57.50%   57.57%   +0.07%     
==========================================
  Files          71       71              
  Lines        2751     2751              
==========================================
+ Hits         1582     1584       +2     
+ Misses       1169     1167       -2     

Files	Coverage Δ
...ingCore/src/policies/learners/flux_approximator.jl	100.00% <100.00%> (ø)
...arningCore/src/policies/learners/target_network.jl	95.83% <80.00%> (ø)

... and 1 file with indirect coverage changes