Convert issue templates to YAML-based structured forms with validation

[Copilot]

Migrates GitHub issue templates from markdown to YAML-based structured forms, adds validation rules, and configures template settings to disable blank issues with contact links for discussions and security advisories.

Changes

• Bug Report Template (bug_report.yml)

  • Structured form with validated fields (description, reproduction, expected/actual behavior)
  • Dropdowns for OS (Windows/Linux/macOS/Other) and .NET version (8.0/9.0/10.0)
  • Optional fields for SPDX document (JSON-rendered), context, and solutions
  • Icon: 🐛

• Feature Request Template (feature_request.yml)

  • Structured form with validated fields (problem statement, solution, use cases)
  • Optional fields for alternatives, context, and implementation ideas
  • Icon: ✨

• Template Configuration (config.yml)

  • Disables blank issues (blank_issues_enabled: false)
  • Contact links: 💬 Discussions, 🔒 Security Advisory

• Removed: bug_report.md, feature_request.md

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update issue templates</issue_title>
<issue_description>The issue templates should be updated to use yaml-based structured forms and validation.

The template configuration should be updated to disable blank issues, and to have contact links for discussions and security advisories.

Standard icons should be added for the contact links and template names.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Update issue templates #105

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