[csharp] Rename v5.0 to netstandard1.3

[jimschubert]

The netstandard1.3 was originally presented to users as v5.0. This was deprecated throughout 4.x, and is now renamed as a hard breaking change. The original implementation was from pre-fork in swagger-codegen. I never understood why v5.0 was chosen because it's not even a documented mapping.

Up for discussion: I'd be happy to fully delete this option and uwp from the csharp generator instead.

Whatever route we take, this v5.0 needs to be removed before the actual .NET 5.0 exits preview.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

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[wing328]

LGTM. I'll update the doc after merging this PR into master:

diff --git a/docs/generators/csharp.md b/docs/generators/csharp.md
index 6928ef5f9e..ebb98caaa8 100644
--- a/docs/generators/csharp.md
+++ b/docs/generators/csharp.md
@@ -23,7 +23,7 @@ sidebar_label: csharp
 |returnICollection|Return ICollection<T> instead of the concrete type.| |false|
 |sortParamsByRequiredFlag|Sort method arguments to place required parameters before optional parameters.| |true|
 |sourceFolder|source folder for generated code| |src|
-|targetFramework|The target .NET framework version.|<dl><dt>**v3.5**</dt><dd>.NET Framework 3.5 compatible</dd><dt>**v4.0**</dt><dd>.NET Framework 4.0 compatible</dd><dt>**v4.5**</dt><dd>.NET Framework 4.5 compatible</dd><dt>**v4.5.2**</dt><dd>.NET Framework 4.5.2+ compatible</dd><dt>**v5.0**</dt><dd>.NET Standard 1.3 compatible (DEPRECATED. Please use `csharp-netcore` generator instead)</dd><dt>**uwp**</dt><dd>Universal Windows Platform (DEPRECATED. Please use `csharp-netcore` generator instead)</dd></dl>|v4.5|
+|targetFramework|The target .NET framework version.|<dl><dt>**v3.5**</dt><dd>.NET Framework 3.5 compatible</dd><dt>**v4.0**</dt><dd>.NET Framework 4.0 compatible</dd><dt>**v4.5**</dt><dd>.NET Framework 4.5 compatible</dd><dt>**v4.5.2**</dt><dd>.NET Framework 4.5.2+ compatible</dd><dt>**netstandard1.3**</dt><dd>.NET Standard 1.3 compatible (DEPRECATED. Please use `csharp-netcore` generator instead)</dd><dt>**uwp**</dt><dd>Universal Windows Platform (DEPRECATED. Please use `csharp-netcore` generator instead)</dd></dl>|v4.5|
 |useCollection|Deserialize array types to Collection&lt;T&gt; instead of List&lt;T&gt;.| |false|
 |useCompareNetObjects|Use KellermanSoftware.CompareNetObjects for deep recursive object comparison. WARNING: this option incurs potential performance impact.| |false|
 |useDateTimeOffset|Use DateTimeOffset to model date-time properties| |false|