.NET: Update version for 1.21.0 release

[SergeyMenshykh]

Motivation & Context

Bump the .NET version for the 1.21.0 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 1.21.0.
• 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]

🟢 Approval recommended

The version, Git tag, and date suffix are consistent with the intended release.

Pull request overview

Updates centralized .NET package metadata for the 1.21.0 release.

Changes:

• Sets version and Git tag to 1.21.0.
• Updates the date suffix to 260911.

File summaries

File	Description
dotnet/nuget/nuget-package.props	Updates centralized release values.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Balanced

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[github-actions]

MAF Automated Review — Iteration 1

Result: No findings
Scope: full PR (1 commit(s)): 3cbf680c0db4
Model: gpt-5.6-sol-fast

Overview

The PR consistently advances the central .NET package version and Git tag to 1.21.0 and sets the preview date suffix to the 2026-09-11 release date. Existing MSBuild conditions preserve the intended GA, RC, explicit-suffix, and preview package variants, while release validation and public API gates remain active. No Critical, High, or Medium defect introduced by the reviewed diff was established.

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.