Dependency compilation should add deprecation dates

[robdimsdale]

Describe the Enhancement

Each version of Python is supported until a specific date (see e.g. https://endoflife.date/python). We can therefore make use of the deprecation_date field in the dependencies metadata. We were previously doing that, but the switch to the new dependencies workflow has caused this field to be dropped.

We should put these dates back to the dependency metadata.

Possible Solution

I think the best place to add this is the [[metadata.dependency-constraints]] object. Each dependency constraint maps to a minor line, and each minor line has a specific, well-defined deprecation date.

Motivation

The deprecation date is useful to humans and automation, and should be included if it is accurate and verifiable.