Obsolete UnixTimestampExtensions methods

[0xced]

• The equivalent of public static DateTimeOffset FromUnixTime(this long unixTime) exists in the framework since .NET Framework 4.6: https://docs.microsoft.com/en-us/dotnet/api/system.datetimeoffset.fromunixtimeseconds
• The equivalent of public static long ToUnixTime(this DateTimeOffset date) exists in the framework since .NET Framework 4.6: https://docs.microsoft.com/en-us/dotnet/api/system.datetimeoffset.tounixtimeseconds

[codecov]

Codecov Report

Merging #2121 into master will decrease coverage by 0.01%.
The diff coverage is 37.5%.

@@            Coverage Diff             @@
##           master    #2121      +/-   ##
==========================================
- Coverage   66.98%   66.96%   -0.02%     
==========================================
  Files         542      542              
  Lines       14239    14238       -1     
==========================================
- Hits         9538     9535       -3     
- Misses       4701     4703       +2

Impacted Files	Coverage Δ
Octokit/Helpers/UnixTimeStampExtensions.cs	0% <0%> (-100%)	⬇️
Octokit/Models/Response/WeeklyHash.cs	0% <0%> (ø)	⬆️
Octokit/Models/Response/WeeklyCommitActivity.cs	64.28% <0%> (ø)	⬆️
Octokit/Http/RateLimit.cs	97.5% <100%> (ø)	⬆️
Octokit/Models/Response/AdditionsAndDeletions.cs	52.63% <100%> (ø)	⬆️
Octokit/SimpleJson.cs	70.15% <50%> (ø)	⬆️

[shiftkey]

A great bit of cleanup - thanks @0xced!

[shiftkey]

advisories: The UnixTimestampExtensions.ToUnixTime methods are marked obsolete because there are equivalent APIs available in .NET Framework 4.6. As these are also intended for internal usage in .NET and should not be imported into the library code, these will be removed in a later update.

[shiftkey]

release_notes: Replaced old UnixTimestampExtensions.ToUnixTime extension methods with inbuild equivalents now that we target .NET Framework 4.6