docs: add Capacitor 9 plugin upgrade guide

[andredestro]

Description

Adds docs/main/updating/plugins/9-0.md, the "Updating plugins to 9.0" guide for Capacitor plugin authors, and registers it in the Upgrade Guides sidebar. Also cross-links it with the app upgrade guide (main/updating/9-0.md) added in the base branch.

Covers plugin-author changes:

• Using the @capacitor/plugin-migration-v8-to-v9 CLI tool, and updating package.json dependencies manually (next in devDependencies, >=9.0.0-alpha.1 in peerDependencies)
• Cordova support becoming optional, including the plugin-specific implication of dropping the unconditional Cordova product from Package.swift
• Updating the plugin's Android Gradle variables (AndroidX, Google Maps, kotlinx-coroutines versions), removing targetSdkVersion, migrating core-ktx to core
• Removing the standalone Kotlin Gradle plugin (AGP 9 bundles Kotlin 2.2.10 natively) and jcenter()
• Giving the plugin a unique Android namespace (AGP 9 defaults android.uniquePackageNames to true)
• Raising the iOS deployment target to 16.0 across podspec, Podfile, and Package.swift, and bumping the capacitor-swift-pm dependency

Change Type

• Fix
• Feature
• Refactor
• Breaking Change
• Documentation
• Other (CI, chores, etc.)

Rationale / Problems Fixed

Plugin authors upgrading to Capacitor 9 need a single reference for what changed in their build/dependency setup, separate from the app-level guide, so the Capacitor CLI and other components can link to it.

Internal Jira reference: https://outsystemsrd.atlassian.net/browse/RMET-5321

Screenshots / Media

N/A

Platforms Affected

• Android
• iOS
• Web

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Project	Deployment	Actions	Updated (UTC)
capacitor-docs	Ready	Preview	Aug 12, 2026 10:58am