[C#][Samples/Templates] Add UTF8 default encoding for deployment scripts

[Batta32]

Purpose

What is the context of this pull request? Why is it being done?
Due to the #2007, the #2130 was made fixing the issue in TypeScript, this PR is to replicate that fix in C#.

Related to #2130

Changes

Are there any changes that need to be called out as significant or particularly difficult to grasp? (Include illustrative screenshots for context if applicable.)
We added the argument -Encoding utf8 for some cmdlets like Out-File and Get-Content.
The scripts that we updated in the Templates and Samples, are the following:

• deploy.ps1
• deploy_cognitive_models.ps1
• update_cognitive_models.ps1

Tests

Is this covered by existing tests or new ones? If no, why not?
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Testing Steps

1. Open a terminal in .\templates\Virtual-Assistant-Template\csharp\Sample\VirtualAssistantSample
2. Execute pwsh.exe -File deployment\scripts\deploy.ps1.
3. Verify that the Encoding in appsettings.json and cognitivemodels.json is UTF8.

Feature Plan

Are there any remaining steps or dependencies before this issue can be fully resolved? If so, describe and link to any relevant pull requests or issues.
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Checklist

General

• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the appropriate tests
• I have updated related documentation

Bots

• I have validated that new and updated responses use appropriate Speak and InputHint properties to ensure a high-quality speech-first experience
• I have replicated language model changes across the English, French, Italian, German, Spanish, and Chinese .lu files and validated that deployment is successful

Deployment Scripts

• I have replicated my changes in the Virtual Assistant Template and Sample projects
• I have replicated my changes in the Skill Template and Sample projects