Update to latest version of GB master.

[SergioEstevao]

Update develop branch to have the latest version of GB master.

Includes #1507 too, for which the GB ref bump was missed somehow.
Also includes WordPress/gutenberg#17937, with which we hope we address wordpress-mobile/WordPress-iOS#11950.

To test:

• Just smoke test the demo app to see if all is running.

Update release notes:

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

[hypest]

👋 @SergioEstevao , do you perhaps have a scenario that we can trigger wordpress-mobile/WordPress-iOS#11950 (comment)? It'd be good to test it while in this PR since it includes the GB side fix (WordPress/gutenberg#17937).

[SergioEstevao]

So the one I detected was in my preformatted PR where a block used a RichText component and didn't set the 'identifier' prop to a correct value. When you tried to merge a block that didn't have the prop set you would get an exception.

But if all blocks have the identifier set properly I don't think you will be able to reproduce any longer.
Unless you remove the identifier in one of the blocks explicitly, for example in the paragraph block.

[hypest]

LGTM!

By the way, I updated the PR description to include a mention to Danilo's
PRand Ella's .multiline attribute fix, for future reference).