Update CI to Xcode 26.6 and Ruby 3.4.9

[wojtekn]

Related issues

• Related to AINFRA-2589

How AI was used in this PR

Claude Code located how CI resolves the Xcode/Ruby versions (the repo uses the IMAGE_ID-from-.xcode-version approach via .buildkite/shared-pipeline-vars), confirmed no hardcoded image IDs or simulator references needed updating, and made the two version-file edits. I reviewed the changes.

Proposed Changes

Bumps CI to the newly available Xcode 26.6 image and moves Ruby to the new default 3.4.9.

• Xcode 26.6: our CI infra now offers this image. Since this repo already adopted the IMAGE_ID: $IMAGE_ID approach, updating .xcode-version is sufficient — .buildkite/shared-pipeline-vars derives IMAGE_ID=xcode-26.6 from it, and all pipeline files pick it up.
• Ruby 3.4.9: fixes the bundle install failure when building native-code gems such as Nokogiri, which has been present since Xcode 26.4 (https://bugs.ruby-lang.org/issues/21629). Ruby is only used here for Fastlane release tooling. Gemfile.lock has no RUBY VERSION pin, so no lockfile change is required. Both Buildkite and GitHub Actions (ruby/setup-ruby with bundler-cache: true) read .ruby-version automatically.

No user-facing changes — CI/tooling only.

Testing Instructions

• CI runs on this branch should schedule on the xcode-26.6 image and succeed, including the Fastlane helper tests and PHP CLI binary build (which runs bundle install).

Pre-merge Checklist

• Green CI on the new image.

[wpmobilebot]

📊 Performance Test Results

Comparing eb53af1 vs trunk

app-size

Metric	trunk	eb53af1	Diff	Change
App Size (Mac)	1317.24 MB	1317.24 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	eb53af1	Diff	Change
load	1096 ms	1117 ms	+21 ms	⚪ 0.0%

site-startup

Metric	trunk	eb53af1	Diff	Change
siteCreation	6500 ms	6496 ms	4 ms	⚪ 0.0%
siteStartup	1856 ms	1859 ms	+3 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)