[UA update] chore: Update User-Agent string

[96malhar]

Issue #, if available: SDK-169882

Description of changes: chore: Update User-Agent string.

Testing Verified that the updated UA string is lib/aws-dotnet-deploy-cli#1.20.3.0 md/cli-args#{cli_command}

• The revision part of the version number is intentionally set to 0 since versioning on NuGet follows semantic versioning consisting of Major.Minor.Patch versions.
• The cli_command can be one of deploy, list-deployments, delete-deployment, deployment-project, server-mode

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 61.44%. Comparing base (48f29b1) to head (82e6ade).

Files	Patch %	Lines
src/AWS.Deploy.CLI/App.cs	83.33%	1 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #824 +/- ## ========================================== + Coverage 61.43% 61.44% +0.01% ========================================== Files 277 277 Lines 10659 10666 +7 Branches 1484 1486 +2 ========================================== + Hits 6548 6554 +6 + Misses 3571 3569 -2 - Partials 540 543 +3 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.