Add deprecation warning for using Rinkeby network

[leszko]

No description provided.

[codecov]

Codecov Report

Merging #2696 (07ca77a) into master (3d4bce7) will increase coverage by 0.00390%.
The diff coverage is 0.00000%.

@@                 Coverage Diff                 @@
##              master       #2696         +/-   ##
===================================================
+ Coverage   56.34804%   56.35194%   +0.00390%     
===================================================
  Files             88          88                 
  Lines          19124       19128          +4     
===================================================
+ Hits           10776       10779          +3     
- Misses          7760        7761          +1     
  Partials         588         588                 

Impacted Files	Coverage Δ
cmd/livepeer/starter/starter.go	2.85189% <0.00000%> (-0.00525%)	⬇️
core/orchestrator.go	77.65517% <0.00000%> (-0.21482%)	⬇️
discovery/db_discovery.go	70.75812% <0.00000%> (+1.08303%)	⬆️

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Impacted Files	Coverage Δ
cmd/livepeer/starter/starter.go	2.85189% <0.00000%> (-0.00525%)	⬇️
core/orchestrator.go	77.65517% <0.00000%> (-0.21482%)	⬇️
discovery/db_discovery.go	70.75812% <0.00000%> (+1.08303%)	⬆️

[yondonfu]

LGTM