Add platform requirements

[d-date]

Add platform requirements to do release build in Apple platforms in Xcode 13.

Motivation:

In Xcode 13 beta, Swift libraries fail to build for iOS targets that use armv7.
This will affect to release build in Apple platforms.

We can reproduce this issue on running archive build on NIOTransportServices scheme.

Modifications:

As described in Xcode release note, just add platforms: in Package.swift.

Swift libraries may fail to build for iOS targets that use armv7. (74120874)
Workaround: Increase the platform dependency of the package to v12 or later.
https://developer.apple.com/documentation/xcode-release-notes/xcode-13-beta-release-notes

This is caused by using Network.framework so platforms versions are depends on Network.framework availability.
The platforms: attribute will be ignored in Linux or Server Side Swift.

Result:

Release build has succeeded.

[swift-server-bot]

Can one of the admins verify this patch?

[swift-server-bot]

Can one of the admins verify this patch?

[swift-server-bot]

Can one of the admins verify this patch?

[swift-server-bot]

Can one of the admins verify this patch?

[swift-server-bot]

Can one of the admins verify this patch?

[glbrntt]

We can't accept this patch as changing platforms: will break existing users who support older platforms. As noted though, this is a known issue in a beta release which may be fixed in a subsequent Xcode version.