docs: add Capacitor 9 upgrade guide with iOS deprecated API removals

[andredestro]

Description

Adds the initial "Updating to 9.0" upgrade guide (docs/main/updating/9-0.md) and registers it in the Upgrade Guides sidebar.

The guide currently covers the iOS portion of the Capacitor 9 release:

• Raising the iOS deployment target to 16.0 (Xcode project and Podfile).
• The full list of Swift and Obj-C APIs removed in Capacitor 9 (deprecated in previous majors), with a replacement table for each: the CAPBridge compatibility class, the deprecated CAPBridgeProtocol methods, the CAPNotifications enum, the PluginCallErrorData/PluginResultData/JSResultBody typealiases, hasOption, JSDate, the legacy InstanceConfiguration accessors, CAPPlugin.getConfigValue:, CAPFileManager, the deprecated CapacitorBridge initializer, httpsInterceptorStartIdentifier and the setRequestHeaders([String: String]) overload.

The guide follows the same structure as the existing 8-0.md.

Change Type

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

Rationale / Problems Fixed

Capacitor 9 removes the iOS APIs that were deprecated in previous major versions (see ionic-team/capacitor#8551). Plugin authors and app developers hitting compile errors after upgrading need a single place that maps each removed API to its replacement (RMET-5303).

This first version of the guide is intentionally scoped to iOS; Android, CLI and config sections can be added by the corresponding Capacitor 9 changes as they land.

Tests or Reproductions

Content was verified against the actual removals in ionic-team/capacitor#8551: every removed symbol listed in the guide matches the source changes, and the replacements were validated by migrating Capacitor's own internal usages and test suite to them.

Platforms Affected

• Android
• iOS
• Web

Notes / Comments

• The setRequestHeaders entry includes a behavior note: the remaining [String: Any] overload sets header values instead of appending them, so repeated keys overwrite the previous value.
• Should be merged together with (or after) refactor(ios)!: remove deprecated Swift and Obj-C APIs for Capacitor 9 capacitor#8551, which lands the removals for the 9.0 release.
• docs: add Android deprecated API removals to Capacitor 9 upgrade guide #581 adds the Android section to this same page; whichever merges second needs a trivial rebase.

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Project	Deployment	Actions	Updated (UTC)
capacitor-docs	Ready	Preview	Aug 5, 2026 3:21pm