Remove NET5 from tests and CI

[cijothomas]

Towards #3147
This PR only touches CI, and Tests which had net5.0 target. (this should make CI jobs take less time!)
There are projects which has net5.0 target, and those will be updated in a follow-up PR.

Also removed net461 with net462 in Zpages, which was left out earlier.

[codecov]

Codecov Report

Merging #3215 (eab7c67) into main (b4267c7) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3215      +/-   ##
==========================================
+ Coverage   85.72%   85.76%   +0.04%     
==========================================
  Files         260      260              
  Lines        9358     9358              
==========================================
+ Hits         8022     8026       +4     
+ Misses       1336     1332       -4     

Impacted Files	Coverage Δ
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	96.87% <0.00%> (-0.79%)	⬇️
...metryProtocol/Implementation/ActivityExtensions.cs	92.34% <0.00%> (+0.90%)	⬆️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	82.35% <0.00%> (+8.82%)	⬆️