Rename backend variables to moi_backend

[odow]

Extracted from #2610

It's a bit confusing having backend as a variable and as a function, and this unifies the usage of moi_add_constraint.

[codecov]

Codecov Report

Merging #2616 (092ea8c) into master (5bdec26) will not change coverage.
The diff coverage is 98.14%.

@@           Coverage Diff           @@
##           master    #2616   +/-   ##
=======================================
  Coverage   93.50%   93.50%           
=======================================
  Files          44       44           
  Lines        5544     5544           
=======================================
  Hits         5184     5184           
  Misses        360      360           

Impacted Files	Coverage Δ
src/variables.jl	98.78% <98.14%> (ø)

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