Update PSReadLine build to target netstandard2.0

[daxian-dbw]

PR Summary

By retargeting PSReadLine to netstandard2.0, we can simplify the build a lot and produce the same assembly no matter building on Windows or non-Windows.

• Update PSReadLine build to target netstandard2.0, including the CI and release pipeline YAML files.
• Update the test to target net472 and net6.0, so that we can run tests with both .NET Framework and .NET
• Rename Microsoft.PowerShell.PSReadLine2.dll to Microsoft.PowerShell.PSReadLine.dll
• Update and clean up the README.md with the up-to-date information

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure you've added one or more new tests Build changes only
• Make sure you've tested these changes in terminals that PowerShell is commonly used in (i.e. conhost.exe, Windows Terminal, Visual Studio Code Integrated Terminal, etc.)
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed at PowerShell-Docs
    • Doc Issue filed:

Microsoft Reviewers: Open in CodeFlow

PR Overview

This PR retargets the PSReadLine build to netstandard2.0 to simplify the build process and ensure consistent output across platforms. Key changes include:

• Updating the build and test commands in various configuration files (README.md, appveyor.yml, pipelines).
• Renaming the assembly from PSReadLine2.dll to PSReadLine.dll.
• Adjusting conditional logic in PSReadLine/OnImportAndRemove.cs to reference a netstandard target instead of net462.

Reviewed Changes

File	Description
README.md	Updated installation, upgrading, and usage instructions.
PSReadLine/OnImportAndRemove.cs	Changed assembly resolution logic to target netstandard framework.
.github/CONTRIBUTING.md	Updated build instructions to refer to the README section.
appveyor.yml	Removed explicit net462 target for Release build; updated test targets.
.pipelines/PSReadLine-Official.yml	Updated build and signing steps to use the new assembly name and targets.

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

PSReadLine/OnImportAndRemove.cs:31

• [nitpick] The literal 'netstd' is ambiguous. Consider renaming it to 'netstandard2_0' or similar for clearer indication of the target framework.

string subd = (Environment.Version.Major >= 6) ? "net6plus" : "netstd";