Update to ably-cocoa v.1.2.10.

[QuintinWillison]

The experience of updating this dependency was even worse this time, than before, and definitely not as easy as the time before that.

So many layers of tooling. This time I ended up doing gem update and also reinstall of cocoapods gem. That may have solved it. Not sure, honestly. The pernicious error I kept getting was:

ios % pod update Ably
Updating local specs repositories
Analyzing dependencies
[!] CocoaPods could not find compatible versions for pod "Ably":
  In Podfile:
    ably_flutter (from `.symlinks/plugins/ably_flutter/ios`) was resolved to 1.2.10, which depends on
      Ably (= 1.2.10)

None of your spec sources contain a spec satisfying the dependency: `Ably (= 1.2.10)`.

You have either:
 * mistyped the name or version.
 * not added the source repo that hosts the Podspec to your Podfile.
?31

TL;DR: Updating the ably-cocoa dependency is a PITA!