Bump macOS minimum version to v11

[marcprux]

This enables ZIPFoundation (and thus the Readium swift-toolkit) to be built from the macOS CLI with swift build. Otherwise, the following errors occur:

zap appfair/ZIPFoundation % swift build
Building for debugging...
/opt/src/github/appfair/ZIPFoundation/Sources/ZIPFoundation/FileManager+ZIP.swift:104:48: error: 'reduce' is only available in macOS 10.15 or newer
 11 | import Foundation
 12 | 
 13 | extension FileManager {
    | `- note: add @available attribute to enclosing extension
 14 | 
 15 |     typealias CentralDirectoryStructure = Entry.CentralDirectoryStructure
    :
 92 |     ///   - pathEncoding: Encoding for entry paths. Overrides the encoding specified in the archive.
 93 |     /// - Throws: Throws an error if the source item does not exist or the destination URL is not writable.
 94 |     public func unzipItem(at sourceURL: URL, to destinationURL: URL,
    |                 `- note: add @available attribute to enclosing instance method
 95 |                           skipCRC32: Bool = false, allowUncontainedSymlinks: Bool = false,
 96 |                           progress: Progress? = nil, pathEncoding: String.Encoding? = nil) async throws {

[mickael-menu]

Hi Marc, thank you for your contribution!

Would you mind disabling the workflows, or fix the Xcode ones that don't seem to run properly? We don't need to support Linux at all for this fork. Thank you