App: fix build error caused by SwiftPM version difference

[clkbng]

My App's Network Extension causes the following error, depends on public remote SwiftPM package.

Invalid manifest

/(path_to)/TemporaryFile.hzdchM.swift:9:17: error: 'v12' is unavailable
.macOS(.v12),
^~~
PackageDescription.SupportedPlatform:18:27: note: 'v12' was introduced in PackageDescription 5.5
public static let v12: PackageDescription.SupportedPlatform.MacOSVersion
^
/(path_to)/TemporaryFile.hzdchM.swift:10:15: error: 'v15' is unavailable
.iOS(.v15)
^~~
PackageDescription.SupportedPlatform:65:27: note: 'v15' was introduced in PackageDescription 5.5
public static let v15: PackageDescription.SupportedPlatform.IOSVersion
^ in https://git.zx2c4.com/wireguard-apple

As error message says, MacOSVersion.v12¹ and IOSVersion.v15² are available in PackageDescription 5.5 and above, but Package.swift still specifies SwiftPM version 5.3.
So, we must bump the swift-tools-version as well.

Footnotes

1. https://developer.apple.com/documentation/packagedescription/supportedplatform/macosversion/v12 ↩

2. https://developer.apple.com/documentation/packagedescription/supportedplatform/iosversion/v15 ↩