Migrate from PackageES build agents to public Microsoft hosted Azure agents

[jefgen]

Summary

This PR updates the build pool used for building the MS-ICU Nuget to migrate off the PackageES build agents and uses the public Microsoft hosted Azure DevOps build agents instead.

It also changes the publishing of the symbols for the MS-ICU Nuget, so that we can continue to have symbols available via the public MSDL symbol server.

PR Checklist

• I have verified that my change is specific to this fork and cannot be made upstream.
• I am making a maintenance related change.
• I am making a change that is related to usage internal to Microsoft.
• I am making a change that is related to the Windows OS build of ICU.
• CLA signed. If not, please see here to sign the CLA.

Detailed Description

The Package ES build agents are being deprecated, and will stop working after Sept 30th. This means that we need to move off of the Package ES CodeHub Lab E pool and find another pool to run our builds on.

Originally, I thought that we'd need to migrate to a custom hosted pool (using the 1ES Hosted Pool to run custom agents), but after doing some investigating, it turns out that we can actually run on the public Microsoft hosted Azure DevOps build agents instead.

This means that we don't need to setup and run our own custom pool in an Azure subscription. 😊

Additionally, we can also continue to publish the Windows symbols for the MS-ICU Nuget to the public MSDL symbol server, by making some changes to how things are published.

Instead of using the AAD authentication to publish, we can switch to using a secure variable for a PAT token via the TARGET Dev Bot (@targetdev-bot) in order to publish the symbols. This variable is only accessible to the manually triggered "Release-Nuget" pipeline, so that other pipelines cannot access it.

So the symbols will still be available from the public MSDL symbol server:
https://msdl.microsoft.com/download/symbols

[jefgen]

Note/FYI: I'll create a separate PR to bump the version number once this is merged.

[huichen123]