Skip InetUtils tests on hosted Mac, target netcoreapp3.1

[TimHess]

Several tests recently started failing on MacOS CI builds though no Steeltoe code changed, this PR skips those on Mac
Also:

• updates dependencies and tests to use netcoreapp3.1 instead of netcoreapp3.0
• .csproj and pipeline maintenance

[codecov]

Codecov Report

Merging #354 into 2.x will increase coverage by 3.54%.
The diff coverage is 58.82%.

@@            Coverage Diff             @@
##              2.x     #354      +/-   ##
==========================================
+ Coverage   71.34%   74.89%   +3.54%     
==========================================
  Files         819      714     -105     
  Lines       23380    20344    -3036     
  Branches     4262     3447     -815     
==========================================
- Hits        16681    15236    -1445     
+ Misses       5530     4186    -1344     
+ Partials     1169      922     -247     

Impacted Files	Coverage Δ
...Common/src/Common.Hosting/HostBuilderExtensions.cs	90.47% <ø> (ø)
...udFoundryCore/CloudFoundryHostBuilderExtensions.cs	55.55% <ø> (ø)
.../ConfigServerConfigurationBuilderExtensionsCore.cs	88.23% <ø> (ø)
...intCore/Env/EndpointServiceCollectionExtensions.cs	69.23% <ø> (ø)
...ngBootAdminClient/BootAdminAppBuilderExtensions.cs	81.63% <ø> (ø)
...yForwarder/EndpointApplicationBuilderExtensions.cs	0.00% <ø> (ø)
...pkin/ZipkinExporterApplicationBuilderExtensions.cs	0.00% <ø> (ø)
...ipkin/ZipkinExporterServiceCollectionExtensions.cs	89.28% <ø> (ø)
...hentication.CloudFoundryBase/CloudFoundryHelper.cs	74.07% <ø> (ø)
...ation.CloudFoundryCore/CloudFoundryOAuthHandler.cs	80.18% <ø> (ø)
... and 130 more

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

72.0% Coverage
0.0% Duplication

[TimHess]

Our test failures seem to be caused by this issue, revisit after azure pipelines get the update (next week?)