fix(templates): use shipped Orleans package version

[ReubenBond]

Problem

The orleans and orleans-web templates hard-code Orleans 10.2.2, so newly generated projects can reference a release older than the installed template package.

Solution

Stage both template manifests through the NuGet pack extension point and set their default Orleans version from $(PackageVersion). The checked-in manifests use the repository's 10.0.0-dev version for local template development, while packed artifacts inherit their exact package version, including prerelease labels.

The pack target validates that each manifest contains exactly one expected development default before replacing it, so manifest drift fails packaging instead of silently shipping a stale version.

The template README now documents that generated projects default to the Orleans version shipped with the template package.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Updates the Orleans project templates packaging so generated projects default to the same Orleans package version as the installed Microsoft.Orleans.Templates NuGet package, avoiding stale hard-coded versions in newly created projects.

Changes:

• Switch the checked-in template manifests’ default Orleans version to a development placeholder (10.0.0-dev) for local template development.
• Add a pack-time MSBuild target which stages the two template manifests and replaces the development placeholder with $(PackageVersion), validating the placeholder appears exactly once per manifest.
• Update the template README to document that generated projects default to the version shipped with the template package (including prerelease versions).

Show a summary per file

File	Description
templates/Microsoft.Orleans.Templates/templates/orleans/.template.config/template.json	Updates the checked-in default Orleans version to the development placeholder used for pack-time replacement.
templates/Microsoft.Orleans.Templates/templates/orleans-web/.template.config/template.json	Updates the checked-in default Orleans version to the development placeholder used for pack-time replacement.
templates/Microsoft.Orleans.Templates/README.md	Clarifies that template output uses the Orleans version shipped in the template package, and that --orleans-version overrides it.
templates/Microsoft.Orleans.Templates/Microsoft.Orleans.Templates.csproj	Adds pack-time staging/rewrite of template manifests via a Roslyn MSBuild task and includes the rewritten manifests in the nupkg.

Review details

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• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Lite