ci: standardise GitHub Actions workflows

[joreilly]

Summary

Standardises the GitHub Actions workflows to a shared fleet-wide baseline (part of a cross-sample CI standardisation).

• Runner: Android/JVM build moved to ubuntu-latest (Android builds don't need macOS — ~10× cheaper minutes). iOS xcodebuild jobs stay on macOS. (Validated: Kotlin/Native compileKotlinIos* compile steps run fine on Linux.)
• Caching: added gradle/actions/setup-gradle@v6 (Gradle build + dependency caching); iOS jobs also cache ~/.konan for Kotlin/Native.
• Action pins: checkout@v7, setup-java@v5, setup-gradle@v6, cache@v6 (fleet-latest majors).
• JDK: left unchanged per repo — the Gradle build pins a Java toolchain, so the runner JDK is kept as-is (only the setup action was bumped).
• Hardening: added permissions: contents: read, timeout-minutes, and concurrency (cancel-in-progress).
• Triggers: normalised to pull_request + push: [main] so the Actions cache seeds from the default branch.

Test plan

• Android CI green on ubuntu-latest
• iOS CI green on macos-latest

🤖 Generated with Claude Code