Create changelog for v1.20.0 release

[Copilot]

This PR creates the changelog for the v1.20.0 release by documenting all user-facing changes merged between v1.19.0 (September 12, 2025) and the current main branch (October 8, 2025).

Changes Made

• Updated CHANGELOG.md with v1.20.0 section containing:
  • 11 Features Added: Service dependencies with uses property, new azd publish command, custom service target support, auto-installation for extensions, Container App Jobs support, Bicep Container App revisions, extensions promoted from alpha to beta, MCP Server capability, agent elicitation support, custom language support, and updated azd ext show UX
  • 13 Bugs Fixed: Multiple fixes for custom service targets, publishing, Aspire deployments, agent UX improvements, role assignment scoping, hook shell defaults, and platform-specific fixes
  • 11 Other Changes: Bicep and CLI tool updates, dependency bumps, CVE fixes, extension registry updates, and documentation improvements
• Updated version.txt from 1.20.0-beta.1 to 1.21.0-beta.1 for the next release cycle
• Converted the "1.20.0-beta.1 (Unreleased)" section to "1.20.0 (TBD)"
• Added new "1.21.0-beta.1 (Unreleased)" section at the top

Validation

• ✅ Spell check passed (cspell)
• ✅ All entries follow existing changelog format and style
• ✅ PRs categorized according to changelog guidelines
• ✅ Test/CI/documentation-only changes excluded per guidelines

The changelog systematically documents all user-facing changes from the v1.20.0 release window, following the same format and guidelines used in previous releases.

Fixes #5887

Original prompt

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This section details on the original issue you should resolve

<issue_title>Create changelog for v1.19.0 release</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are Copilot in this section)

Fixes #5887

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[Rajesh Kamal (rajeshkamal5050)]

Copilot looks like issue #5887 was filed asking for changelog v1.19.0 which is already completed. The issue is now updated to Create changelog for v1.20.0 release. Can you update the PR for v1.20.0?

[Copilot]

Copilot looks like issue #5887 was filed asking for changelog v1.19.0 which is already completed. The issue is now updated to Create changelog for v1.20.0 release. Can you update the PR for v1.20.0...

Updated the PR for v1.20.0. The changelog now includes 11 features, 13 bug fixes, and 11 other changes from commits between Sep 12 and Oct 8, 2025. Version.txt updated to 1.21.0-beta.1 for the next release cycle.