Update nuget packages content

- add buildMultitargeting
- remove buildTransitive/netcoreapp3.1 and add net5.0, net6.0
- update readme files

Author: alex-kulakov

Orm/Xtensive.Orm.Firebird/NuGetContent/ReadMe.md

@@ -2,7 +2,7 @@
 
 The provider is responsible for interactions with Firebird database - information about features and types the storage supports, low level communications, translation of SqlDom queries to native SQL text queries.
 
-For now it supports Firebird 2.5.
+For now it supports Firebird 2.5, 4.0.
 
 ### Usage
 

Orm/Xtensive.Orm.MySql/NuGetContent/ReadMe.md

@@ -2,7 +2,7 @@
 
 The provider is responsible for interactions with MySQL database - information about features and types the storage supports, low level communications, translation of SqlDom queries to native SQL text queries.
 
-For now it supports MySQL 5.5, 5.6
+For now it supports MySQL 5.6, 5.7, 8.0.
 
 ### Usage
 

Orm/Xtensive.Orm/NuGetContent/ReadMe.md

@@ -7,8 +7,8 @@ Supported databases:
 - MS Azure SQL Database
 - Oracle 10g, 11g
 - PostgreSQL 8.3, 8.4, 9.x, 10, 11, 12, 13, 14, 15
-- MySQL 5.5, 5.6
-- Firebird 2.5
+- MySQL 5.6, 5.7, 8.0
+- Firebird 2.5, 4.0
 - Sqlite 3
 
 Providers for the databases are available as separate packages and may be installed following way

Orm/Xtensive.Orm/NuGetContent/buildTransitive/netcoreapp3.1/Xtensive.Orm.targets renamed to Orm/Xtensive.Orm/NuGetContent/buildMultitargeting/net5.0/Xtensive.Orm.targets

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+
+<!--
+  Copyright (C) 2011-2013 Xtensive LLC.
+  All rights reserved.
+  For conditions of distribution and use, see license.
+-->
+
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+<Import Project="$(MSBuildThisFileDirectory)..\..\DataObjects.Net.targets" />
+
+</Project>

Orm/Xtensive.Orm/NuGetContent/buildMultitargeting/net6.0/Xtensive.Orm.targets

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+
+<!--
+  Copyright (C) 2011-2013 Xtensive LLC.
+  All rights reserved.
+  For conditions of distribution and use, see license.
+-->
+
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+<Import Project="$(MSBuildThisFileDirectory)..\..\DataObjects.Net.targets" />
+
+</Project>

Orm/Xtensive.Orm/NuGetContent/buildTransitive/net5.0/Xtensive.Orm.targets

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+
+<!--
+  Copyright (C) 2011-2013 Xtensive LLC.
+  All rights reserved.
+  For conditions of distribution and use, see license.
+-->
+
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+<Import Project="$(MSBuildThisFileDirectory)..\..\DataObjects.Net.targets" />
+
+</Project>