Adopt flutter_plugin_tools podspecs linter

[jmagman]

lint_darwin_plugins.sh now lives in flutter_plugin_tools and has more linting features.
Already done in Flutter plugins with flutter/plugins#2346.

[jmagman]

lint_darwin_plugins.sh no longer exists and the podspec linting has moved from .cirrus.yml to https://github.com/FirebaseExtended/flutterfire/blob/584d0d340bae7ed3f32c09fe68aab7ddf32a4229/melos.yaml#L75

[darshankawar]

Hi @Salakar
Couldn't find a relevant label for this. Should this be kept as-is ?
Thanks.

[Lyokone]

I'm closing this for the following reasons:

• We do not publish the podspecs so the warning are not relevant (missing author or license are included in the dart package)
• the underlying command pod lib lint does not support importing variables from ruby files which we use for defining a global firebase_sdk_version

Anybody feel free to reopen if you feel it's important