Remove LinuxImageDemand and WindowsImageDemand variables, replace hardcoded ubuntu-24.04 with $(LINUXVMIMAGE)

[Copilot]

Summary

This PR removes the deprecated LinuxImageDemand and WindowsImageDemand variables from pipeline configuration files and replaces hardcoded ubuntu-24.04 references with the standardized $(LINUXVMIMAGE) variable. This aligns with the changes made in Azure/azure-sdk-for-net#50802 and follows the latest pipeline configuration requirements.

Changes Made

Variable Removal

• Removed LinuxImageDemand variable definition from /eng/pipelines/templates/variables/image.yml
• Confirmed no WindowsImageDemand variables existed in the codebase

Pipeline Updates

• Updated /eng/pipelines/docindex.yml:

  # Before
  demands: $(LinuxImageDemand)
  
  # After  
  demands: ImageOverride -equals $(LINUXVMIMAGE)

Hardcoded Value Replacements

Replaced hardcoded ubuntu-24.04 with $(LINUXVMIMAGE) in the following files:

• /eng/common/pipelines/codeowners-linter.yml
• /eng/common/pipelines/templates/stages/archetype-sdk-tool-pwsh.yml
• /eng/common/pipelines/templates/jobs/archetype-sdk-tests-generate.yml

Files Intentionally Unchanged

• npm-publish.yml: Left unchanged due to deployment job constraints (documented in comments explaining hardcoding is necessary)
• image.yml variable definitions: Correctly define LINUXVMIMAGE value as ubuntu-24.04
• GitHub workflows: Use GitHub Actions syntax, not Azure DevOps variables

Validation

• ✅ All modified YAML files validated for syntax correctness
• ✅ Created and ran verification tests to ensure proper variable usage
• ✅ Confirmed consistent usage of ImageOverride -equals $(LINUXVMIMAGE) pattern
• ✅ Verified no remaining references to removed variables exist

Impact

This change standardizes pipeline image references across the repository and removes deprecated variables while maintaining full backward compatibility. Pipelines will continue to use the same underlying Ubuntu 24.04 image but now reference it through the centralized $(LINUXVMIMAGE) variable.

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