[CLI] Add LTS Ubuntu 26.04 support for Azure Compute

[Alex-AZPS]

Description

Add support for the new LTS Ubuntu 26.04 version in Azure Compute.

This change updates the supported Ubuntu LTS version list to include Ubuntu 26.04, enabling users to use the latest Ubuntu LTS image version with Compute commands.

Changes Made

• Add Ubuntu 26.04 as a supported LTS Ubuntu version.
• Update related validation/configuration to recognize the new version.
• Ensure the new version is available in relevant Compute scenarios.

Testing

• Verified the new Ubuntu 26.04 version is accepted by validation.
• Tested related Compute workflows to ensure no regression.

Related Issue

[yonzhan]

Thank you for opening this issue, we will look into it.

[microsoft-github-policy-service]

🔔 Routing this issue to @Azure/act-observability-squad.

[microsoft-github-policy-service]

🔔 Routing this issue to @Azure/act-observability-squad.

[microsoft-github-policy-service]

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @dkkapur, @Azure/act-observability-squad.