Support retrieving and updating generic release-level data, such as support and eol dates. The JSON format has been changed accordingly to add a new top-level `releases` key.
The `aws-lambda.py` script has been updated to make use of this new feature.
This makes the format open for extension, such as adding release cycle level data (such as EOL dates).
Version data is still accessible by the version's name. While this repeats the version name, it's also much more convenient for users of those data.
A few other things have also been updated in the process:
- verbosity of the diff has been increased in update.py to make workflow summaries more readable,
- dates without timezone are now set to UTC by default (this was already supposed, so no impact expected here).
