Create changelog for v1.20.1 release

[Copilot]

This PR creates the changelog for the v1.20.1 release, documenting all changes since v1.20.0 (released on October 8, 2025).

Changes

Version Updates

• Updated cli/azd/CHANGELOG.md header from 1.21.0-beta.1 (Unreleased) to 1.20.1 (2025-10-17)
• Updated cli/version.txt from 1.21.0-beta.1 to 1.20.1

Changelog Entries

The v1.20.1 release includes 4 new features and 2 bug fixes:

Features Added:

• GitHub Copilot LLM Integration (Add GitHub Copilot LLM Backend Integration #5693): Added GitHub Copilot as a new LLM backend for AI-assisted development workflows, enabling seamless authentication and token management for azd agent features
• Extension Framework Expansion (Add support for language frameworks from extensions #5847): Added support for custom language frameworks from extensions, allowing extensions to provide language-specific build and package operations via gRPC
• Aspire Dashboard Discoverability (Added support to print app service aspire dashboard url #5881): Automatically display Aspire dashboard URL for App Service deployments, making it easier for customers to discover and monitor compute resources
• Schema Enhancement (Add prepublish and postpublish hooks to v1.0 azure.yaml schema #5841): Added prepublish and postpublish hooks to the v1.0 azure.yaml schema for better lifecycle management

Bugs Fixed:

• Container App Deployment (containerapp: fix check for resource exists #5939): Fixed Container App resource existence check that was incorrectly returning true regardless of actual resource state
• Debug Message Clarity (Soften diagnostic message for missing --environment flag #5930): Improved diagnostic message for missing --environment flag to clearly indicate expected fallback behavior

Methodology

The changelog was created by:

1. Identifying the cutoff commit from the v1.20.0 release (cd0da10)
2. Reviewing all commits merged to main between October 9-16
3. Fetching PR and linked issue details for each relevant commit
4. Categorizing changes as features, bug fixes, or other changes
5. Excluding internal refactoring, test infrastructure, and extension-specific changes without user impact
6. Validating formatting and running spell check

Fixes #5940

Original prompt

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This section details on the original issue you should resolve

<issue_title>Create changelog for v1.20.1 release</issue_title>
<issue_description></issue_description>

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

Fixes #5941

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[hemarina]

/check-enforcer override