Update Gutenberg to 1.6.0

[hypest]

This PR updates the gutenberg-mobile reference to point to the latest release, v1.6.0.

The reference is currently pointing to the release/1.6.0 branch on https://github.com/wordpress-mobile/gutenberg-mobile. Relevant PR: wordpress-mobile/gutenberg-mobile#1036.

To test:
The block editor should work as normal.

Update release notes:

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

[hypest]

FYI, produced the changes in 61a503c by manually editing the Podfile to point to the gutenberg-mobile commit hash and then ran pod update in the command line.

[hypest]

We found some issues with the video and the quote blocks so, we've decided over Slack to hold back the release, put the video, quote blocks behind the __DEV__ flag to only have them on debug builds, and then do the code freeze.

[koke]

Found some issues that I mentioned on Slack:

• Video seems to have no sound. Known issue at When muted={true} the fullscreen volume control won't unmute TheWidlarzGroup/react-native-video#1192
• The setting icon on the video block seems to do nothing.
• Inserting a quote freezes the app. On relaunch, focusing on the quote leaves the keyboard flickering and unresponsive.

[CHaNGeTe]

Video seems to have no sound. Known issue at TheWidlarzGroup/react-native-video#1192

Try this PR:
TheWidlarzGroup/react-native-video#1654