.NET: Update version for 1.18.0 release

[SergeyMenshykh]

Motivation & Context

Bump the .NET version for the 1.18.0 release.

Description & Review Guide

• What are the major changes? Updates the version, Git tag, and date suffix.
• What is the impact of these changes? .NET packages will use version 1.18.0.
• 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 the central .NET package configuration for the 1.18.0 release.

Changes:

• Sets package and Git tag versions to 1.18.0.
• Advances the preview date suffix to 260818.

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[github-actions]

MAF Automated Review — Iteration 1

Result: No findings
Scope: full PR (1 commit(s)): 12a4b836e7a2
Model: gpt-5.6-sol

Overview

The PR consistently advances the central .NET package version and Git tag to 1.18.0 and updates the preview date suffix to the review date. The conditional package-version rules, explicit project maturity flags, prior release pattern, and evaluated GA, RC, and preview outputs support the change. No publishable Critical, High, or Medium concern 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.