Remove .NET Core install templates from pipelines (Azure#5968)

I believe we can finally remove the .NET Core install templates from our pipelines because either:
1. The pipelines are running on 1ES-hosted agents, which already have the latest version of .NET Core 2.1 and 3.1 installed
2. The pipelines are running on MS-hosted agents, which [already have recent version of .NET Core 2.1 and 3.1 installed](https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu1804-Readme.md#net-core-sdk)

This change removes the templates and their references. The templates were installing a hard-coded, much older version of the SDK, and therefore missed the latest security patches and other fixes. I also updated the Functions binding project to replace some deprecated properties that were causing errors when built against the latest .NET Core SDK.

Note that this PR does not remove the template that configures the default SDK on Linux.

I tested the three pipelines impacted by this change: dotnet checkin, dotnet CI, and images release.

## Azure IoT Edge PR checklist:

This checklist is used to make sure that common guidelines for a pull request are followed.

### General Guidelines and Best Practices
- [X] I have read the [contribution guidelines](https://github.com/azure/iotedge#contributing).
- [X] Title of the pull request is clear and informative.
- [X] Description of the pull request includes a concise summary of the enhancement or bug fix.

### Testing Guidelines
- [X] Pull request includes test coverage for the included changes.
- Description of the pull request includes 
	- [X] concise summary of tests added/modified
	- [X] local testing done.

builds/checkin/dotnet.yaml

@@ -32,7 +32,6 @@ jobs:
  pool:
  vmImage: "ubuntu-18.04"
  steps:
- - template: ../templates/install-dotnet3.yaml
  - task: Bash@3
  displayName: Install Prerequisites
  inputs:

builds/ci/dotnet.yaml

@@ -30,7 +30,6 @@ jobs:
  IotDevice3ConnStr2,
  IotHubConnStr2,
  IotHubMqttHeadCert
- - template: ../templates/install-dotnet3.yaml
  - task: Bash@3
  displayName: Install Prerequisites
  inputs:

builds/misc/images-release.yaml

@@ -12,15 +12,11 @@ stages:
  dependsOn: []
  jobs:
  - job: BuildDotnetComponents
- # This job is run on a hosted linux agent, which needs dotnet 2 and 3 installed as a manual step. 
  # The code sign steps will fail unless we explicitly say to use dotnet 2 before.
  # This means we have to toggle back and forth between primary dotnet installations for the sign and build.
  # TODO: Investigate why we have to toggle primary installs on linux, when we didn't have to do this on windows (now removed).
  displayName: Build Dotnet Components
  steps: 
- # Dotnet 2 needed for codesign
- - template: ../templates/install-dotnet2.yaml
- - template: ../templates/install-dotnet3.yaml
  - template: ../templates/dotnet3-globaljson.yaml # use dotnet 3 as primary install for build
  # Build
  - task: ShellScript@2

builds/templates/dotnet2-globaljson.yaml

@@ -4,4 +4,4 @@ steps:
  displayName: 'Use dotnet 2'
  inputs:
  targetType: 'inline'
- script: 'dotnet new globaljson --force --sdk-version 2.2.207'
+ script: 'dotnet new globaljson --force --sdk-version 2.1'

builds/templates/dotnet3-globaljson.yaml

@@ -4,4 +4,4 @@ steps:
  displayName: 'Use dotnet 3'
  inputs:
  targetType: 'inline'
- script: 'dotnet new globaljson --force --sdk-version 3.1.202'
+ script: 'dotnet new globaljson --force --sdk-version 3.1'

edge-modules/functions/binding/src/Microsoft.Azure.WebJobs.Extensions.EdgeHub/Microsoft.Azure.WebJobs.Extensions.EdgeHub.csproj

@@ -18,14 +18,20 @@
  <Title>Microsoft Azure Edge Function extension</Title>
  <Authors>Microsoft</Authors>
  <PackageRequireLicenseAcceptance>true</PackageRequireLicenseAcceptance>
- <PackageLicenseUrl>https://github.com/Azure/iotedge/blob/master/LICENSE</PackageLicenseUrl>
+ <PackageLicenseFile>LICENSE</PackageLicenseFile>
  <Description>This package contains binding extensions for IoTEdge.</Description>
  <PackageIconUrl>http://go.microsoft.com/fwlink/?LinkID=288890</PackageIconUrl>
+ <PackageIcon>icon.png</PackageIcon>
  <PackageProjectUrl>https://github.com/Azure/iotedge</PackageProjectUrl>
  <Copyright>© Microsoft Corporation. All rights reserved.</Copyright>
  <PackageTags>IoT Microsoft Azure IoTEdge Functions</PackageTags>
  </PropertyGroup>
 
+ <ItemGroup>
+ <None Include="..\..\..\..\..\LICENSE" Pack="true" PackagePath=""/>
+ <None Include="images\icon.png" Pack="true" PackagePath=""/>
+ </ItemGroup>
+
  <ItemGroup>
  <PackageReference Include="Microsoft.Azure.Devices.Client" Version="1.36.3" />
  <PackageReference Include="Microsoft.Azure.WebJobs" Version="3.0.27" />