software: add initial_release_date field, automatically check that it is older than 4 months

[nodiscc]

https://github.com/awesome-selfhosted/awesome-selfhosted-data/blob/master/.github/PULL_REQUEST_TEMPLATE.md states:

Any software project you are adding was first released more than 4 months ago.

Currently, reviewers have to manually look for the date of first release, check that it is older than 4 months, and approve/reject contributions based on the results of these manual checks. This is tedious and error-prone (awesome-selfhosted/awesome-selfhosted#4051)

I suggest adding a mandatory field initial_release_date to all software items, check for the presence of this field, check that the value of this field is at least 4 months in the past, and throw an error otherwise. These checks would have to be implemented in https://github.com/nodiscc/hecat/blob/master/hecat/processors/awesome_lint.py

In the first time, such a missing field would only trigger a warning (not an error), since we first have to manually enter this information for all existing projects (for Github projects this information could probably be added in batch using the API).

Wikipedia's infoboxes on software pages (eg [1] work in the same way/include this information which is manually entered by contributors.

# date of the initial (first) release, in 'YYYY-MM-DD' format
initial_release_date: '2021-02-17'