.NET: Updating dotnet version for release.

[alliscode]

Motivation & Context

Bumping version for dotnet release.

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

Bumps the .NET NuGet package version metadata to prepare for the next release train, updating the central version prefix and corresponding git tag/version suffix inputs used for package version generation.

Changes:

• Updated VersionPrefix from 1.14.0 to 1.15.0.
• Updated DateSuffix to 260722 for preview/suffixed package version generation.
• Updated GitTag to 1.15.0 to align with the new version prefix.

Show a summary per file

File	Description
dotnet/nuget/nuget-package.props	Bumps central .NET package version metadata (VersionPrefix/DateSuffix/GitTag) for the 1.15.0 release line.

Review details

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