Update to iOS11 and Swift 5.

[SergioEstevao]

This PR just updates the GB projects to have a minimum requirements of iOS 11 and to use Swift 5.

To test:

• Check if project builds
• Smoke test the demo app to see if all is correct.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[etoledom]

Looks good!
Thank you 🙏

[koke]

This is being reverted in #1426. When I tried to update WPiOS to the new develop, I found two build issues.

The first is that this PR removed an #available(iOS 11.0, *) check in the aztec wrapper, but RNTAztecView.podspec still has a target of iOS 10. That one has an easy fix: just update s.platform on that podspec.

The second one, I got some errors building Gutenberg.framework about libxml2 not finding a header file. I investigated a bit but I couldn't make sense of the error:

<module-includes>:1:9: note: in file included from <module-includes>:1:
#import "libxml/HTMLparser.h"
        ^
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.1.sdk/usr/include/libxml2/libxml/HTMLparser.h:15:10: error: 'libxml/xmlversion.h' file not found
#include <libxml/xmlversion.h>
         ^
<unknown>:0: error: could not build Objective-C module 'libxml2'

[etoledom]

I got some errors building Gutenberg.framework about libxml2 not finding a header file. I investigated a bit but I couldn't make sense of the error:

@koke - I think we should also bump the s.swift_version = '5.0' on RNAztec, that should fix the libxml2 error.