Bump CI default version of Go to 1.20

[MrAlias]

Developers likely use the latest release of Go in development and we want to catch any bugs (e.g. lint errors, race detection) with the this release before they are merged. Therefore, update the default version the CI system uses to be the latest.

In the process of updating, this also removes use of deprecated math/rand functionality. This functionality was deprecated in Go 1.20.

[codecov]

Codecov Report

Merging #3733 (cac8403) into main (d68b05f) will increase coverage by 1.6%.
The diff coverage is 0.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3733     +/-   ##
=======================================
+ Coverage   79.7%   81.3%   +1.6%     
=======================================
  Files        171     166      -5     
  Lines      12709   12451    -258     
=======================================
  Hits       10130   10130             
+ Misses      2363    2105    -258     
  Partials     216     216             

Impacted Files	Coverage Δ
...orters/otlp/otlpmetric/internal/otest/collector.go	6.9% <0.0%> (+<0.1%)	⬆️
...ters/otlp/otlptrace/internal/otlptracetest/data.go
.../otlp/otlptrace/internal/otlptracetest/otlptest.go
metric/global/global.go
...otlp/otlptrace/internal/otlptracetest/collector.go
...rs/otlp/otlptrace/internal/otlptracetest/client.go

[MrAlias]

This is blocking development efforts, going to merge this slightly earlier than 1 full day.