.NET: Update version for 1.20.0 release

[SergeyMenshykh]

Motivation & Context

Bump the .NET version for the 1.20.0 release.

Description & Review Guide

• What are the major changes? Updates the version prefix, Git tag, and date suffix.
• What is the impact of these changes? .NET packages will use version 1.20.0.
• What do you want reviewers to focus on? Confirm the release values.

Related Issue

N/A

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

Updates centralized .NET package metadata for the 1.20.0 release.

Changes:

• Sets version prefix and Git tag to 1.20.0.
• Updates the date suffix to 260831.

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[github-actions]

MAF Automated Review — Iteration 1

Result: No findings
Scope: full PR (1 commit(s)): fc7c2e2a09be
Model: gpt-5.6-sol

Overview

The PR consistently advances the central .NET package prefix and Git tag to 1.20.0 and updates the preview date suffix to 260831. Existing MSBuild conditions produce the expected GA, RC, named prerelease, and default preview versions, while .NET CI packs and consumes representative packages. No publishable Critical, High, or Medium defect was established in the changed behavior.

Reviewed the supplied pull-request change set across correctness, security/reliability, architecture, and failure behavior.
No publishable findings remained after source verification for this scope.