Drop support for Xcode 15.4

[AlexNachbaur]

Drop Xcode 15.4 support.

• Also cleans up the README and other CI job validation steps that have been problems in the past
• This will unblock #248 which is having build failures due to false-negative problems in Xcode 15.4

[Copilot]

Pull Request Overview

This PR drops support for Xcode 15.4 and moves to require Xcode 16.0 or later, aligning with Apple's App Store submission requirements. The change simplifies the Swift version support by removing the separate Swift 5.10 package manifest and consolidating toolchain requirements.

• Updates minimum Xcode requirement from 15.4 to 16.0
• Removes the dedicated Package@swift-5.10.swift file and consolidates Swift support
• Updates CI workflows and documentation to reflect new version requirements

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
README.md	Updates documentation with new version compatibility matrix and support policy details
Package@swift-5.10.swift	Removes the dedicated Swift 5.10 package manifest file
Package.swift	Adds Swift 5 language mode support and updates platform availability checks
.swift-format	Adds configuration for blank line indentation
.spi.yml	Removes experimental documentation parameters
.github/workflows/tests.yaml	Updates CI to use macOS 15 and simplifies test execution
.github/workflows/documentation-ghpages.yaml	Removes experimental documentation flags
.github/actions/setup-swift/versions.yaml	Updates Swift 5.10 to use Xcode 16.4 with version arguments
.github/actions/setup-swift/action.yaml	Adds support for Swift version arguments in CI

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