remove netstandard1.6 replace with netstandard2.0

[hahn-kev]

This PR attempts to remove support for netstanard1.6 as this requires a custom SortKey class which triggers down stream issues with a number of projects. The main cause of this is as follows:
SIL.WritingSystems netstandard2.0 version uses the netstandard1.6 version of this library, this means SIL.WritingSystems includes references to Icu.SortKey (here). When an App (or Tests) targets net8.0 and adds an explicit reference to icu.net (required for the config file), the app gets the net8.0 version of ICU, this version does not include the Icu.SortKey class. This means when trying to call IcuRulesCollator.GetSortKey you will get the following error at runtime:

  Error Message:
   System.TypeLoadException : Could not load type 'Icu.SortKey' from assembly 'icu.net, Version=2.10.0.0, Culture=neutral, PublicKeyToken=416fdd914afa6b66'.
  Stack Trace:
     at SIL.WritingSystems.IcuRulesCollator.GetSortKey(String source)
   at SIL.WritingSystems.Tests.IcuRulesCollationDefinitionTests.Collation_GetSortKeyWorks() in C:\dev\libpalaso\SIL.WritingSystems.Tests\IcuRulesCollationDefinitionTests.cs:line 96
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

This could probably be solved by always including SortKey, however I don't see the need to support netstandard1.6. netstandard2.0 drops support for .net core 1.0 and 1.1, Mono 4.6, and a couple old Xamarin versions. See table here

This change is related to sillsdev/libpalaso#1330 and sillsdev/liblcm#306

[github-actions]

Test Results

       5 files  ±0     403 suites  ±0   5s ⏱️ -1s
   431 tests ±0     427 ✔️ ±0    4 💤 ±0  0 ❌ ±0 
2 193 runs  ±0  2 095 ✔️ ±0  98 💤 ±0  0 ❌ ±0 

Results for commit 63cbe25. ± Comparison against base commit b7b0666.

[ermshiperete]

Mostly LGTM. Also needs a line in CHANGELOG.md (I guess this is a breaking change).

[hahn-kev]

@ermshiperete I added a changelog summary and removed the ReliabilityContract attributes from non framework builds.

[ermshiperete]

Since this is a breaking change, the commit message needs a line +semver:major so that the major version number gets bumped.

[ermshiperete]

(After the fact: this should be ### Removed)

[ermshiperete]

See #203 for follow-up.