build: Fix lint/vet

[Eric-Warehime]

Summary

It looks like make lint is failing on a few things--not sure how some of these ever passed CI, but here are the fixes.

• The fmt string in conduit/plugins/processors/filterprocessor/gen/generate.go is being skipped for linting, but go vet is failing. Use bytes.Buffer.WriteString instead of fmt.Fprint.
• There are leftover references to appCall in handlers_e2e_test.go where appCall was commented out for the refactoring work. Comment these out.
• It looks like we missed a conversion to v2 of the SDK in api/util_test.go.

[codecov]

Codecov Report

Merging #1445 (4400f6a) into develop (451d4b3) will increase coverage by 0.02%.
The diff coverage is 86.66%.

@@             Coverage Diff             @@
##           develop    #1445      +/-   ##
===========================================
+ Coverage    65.13%   65.15%   +0.02%     
===========================================
  Files           80       80              
  Lines        11376    11377       +1     
===========================================
+ Hits          7410     7413       +3     
+ Misses        3394     3393       -1     
+ Partials       572      571       -1     

Impacted Files	Coverage Δ
...plugins/processors/filterprocessor/gen/generate.go	0.00% <0.00%> (ø)
idb/idb.go	69.35% <ø> (ø)
api/converter_utils.go	92.65% <100.00%> (+0.03%)	⬆️
api/handlers.go	75.60% <100.00%> (+0.13%)	⬆️
idb/postgres/postgres.go	65.35% <100.00%> (ø)

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