Estimate cost using a framerate of 60 rather than 120

[thomshutt]

What does this pull request do? Explain your changes. (required)
We've found that in reality, no assets we receive have a framerate of 120 and so as a stopgap improvement until we start pulling the true FPS, we're going to calculate our estimate with a framerate of 60 instead.

Specific updates (required)

• Change cost estimation default framerate from 120 -> 60

How did you test each of these updates (required)

• Ran test suite

Does this pull request close any open issues?
No

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[leszko]

@thomshutt Note the CI Test failures. Please check it before merging.

[codecov]

Codecov Report

Merging #2896 (521f3c2) into master (32d5d45) will increase coverage by 0.01509%.
Report is 1 commits behind head on master.
The diff coverage is 73.80952%.

@@                 Coverage Diff                 @@
##              master       #2896         +/-   ##
===================================================
+ Coverage   56.45344%   56.46853%   +0.01509%     
===================================================
  Files             89          89                 
  Lines          19416       19448         +32     
===================================================
+ Hits           10961       10982         +21     
- Misses          7849        7858          +9     
- Partials         606         608          +2     

Files	Coverage Δ
core/accounting.go	100.00000% <100.00000%> (ø)
server/broadcast.go	77.97571% <100.00000%> (+0.01785%)	⬆️
server/rpc.go	72.10682% <100.00000%> (ø)
server/segment_rpc.go	78.48837% <100.00000%> (+0.12580%)	⬆️
core/orchestrator.go	76.58999% <26.66667%> (-1.12665%)	⬇️

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Files	Coverage Δ
core/accounting.go	100.00000% <100.00000%> (ø)
server/broadcast.go	77.97571% <100.00000%> (+0.01785%)	⬆️
server/rpc.go	72.10682% <100.00000%> (ø)
server/segment_rpc.go	78.48837% <100.00000%> (+0.12580%)	⬆️
core/orchestrator.go	76.58999% <26.66667%> (-1.12665%)	⬇️