Modernize the Swift CI tutorial (current Swift versions, refreshed pins, cross-platform guidance)

[leogdion]

Why:

Closes: #44868

The "Building and testing Swift" tutorial had aged out of date — examples pinned Swift 5.2/5.3/5.3.3 and referenced swift-actions/setup-swift at a 2021 commit, and the page covered only Ubuntu/macOS swift build / swift test. This PR refreshes the tutorial and adds current cross-platform guidance.

What's being changed (if available, include any code snippets, screenshots, or gifs):

File: content/actions/tutorials/build-and-test-code/swift.md

• Updated Swift versions — matrix → ["6.1", "6.2", "6.3"], single version → "6.3" (current GA release).
• Refreshed the swift-actions/setup-swift pin from the 2021 commit to v2.4.0 (7ca6abe…), keeping the existing not-certified-by-GitHub notice and SHA-pinning reusables.
• New section: Building and testing on Linux with a container — uses the official swift release images, and links the swiftlang/swift nightly images, with a nightly example.
• New section: Building and testing for Apple platforms with Xcode — native xcodebuild + simulator -destination (no third-party action), notes how to check the runner's available Xcode/SDKs/simulators, and how to download a missing simulator runtime (xcodebuild -downloadPlatform).
• New section: Building and testing on Windows — uses compnerd/gha-setup-swift (eeda069…, v0.4.0).
• New section: Building and testing on Android — uses skiptools/swift-android-action (2044b79…, v2.9.5).
• New section: Building and testing across multiple platforms with one action — brightdigit/swift-build (b0a63a8…, v1.5.7), shown as a wrapper over the platform setup above, with per-platform examples (Apple, Linux, Windows, Android, WebAssembly) and the download-platform input.

All third-party examples carry the page's standard actions-not-certified-by-github-comment notice and are SHA-pinned per actions-use-sha-pinning-comment.

Disclosure

I'm the maintainer of brightdigit/swift-build. I've kept its mention neutral and placed it after the native/per-platform approaches (Xcode, compnerd/gha-setup-swift, skiptools/swift-android-action), in the same spirit the page already references swift-actions/setup-swift. If the team prefers a lighter touch, I'm glad to trim the multi-platform section down to a single neutral mention, or drop it entirely and keep just the version/pin/Linux modernization.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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Section outline after this change

• Introduction / Prerequisites / Using a Swift workflow template (unchanged)
• Specifying a Swift version → Using multiple Swift versions / Using a single specific Swift version (versions + pin refreshed)
• Building and testing your code (versions + pin refreshed)
• Building and testing on Linux with a container (new)
• Building and testing for Apple platforms with Xcode (new)
• Building and testing on Windows (new)
• Building and testing on Android (new)
• Building and testing across multiple platforms with one action (new) → Apple platforms / Linux / Windows / Android / WebAssembly

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Source	Review	Production	What Changed
actions/tutorials/build-and-test-code/swift.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17

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[heckj]

A minor re-wording bit that I spotted early on.

I'm not sure that I'd (personally) suggest that including lots of platform builds from a single workflow is a good idea - I've found it far easier to break it up so that I can find and identify errors where re-running something too try again doesn't require all the resources of all platforms. But that's totally my own opinion.