Query the"whattrainisitnow" release schedule API instead of hard-coding scheduled release dates

[gabrielBusta]

This issue pertains to the merge duty step outlined here.

Currently, we manually retrieve the LAST and NEXT known dates from the "whattrainisitnow" service and hard-code them into our configuration file:

shipit/api/src/shipit_api/common/config.py

Lines 57 to 65 in 01cc5f5

	# The next 6 dates are information about the current and next release
	# They must be updated at the same time as FIREFOX_NIGHTLY
	# They can be found on https://whattrainisitnow.com/calendar/
	LAST_SOFTFREEZE_DATE = "2024-04-11"
	LAST_MERGE_DATE = "2024-04-15"
	LAST_RELEASE_DATE = "2024-04-16"
	NEXT_SOFTFREEZE_DATE = "2024-05-09"
	NEXT_MERGE_DATE = "2024-05-13"
	NEXT_RELEASE_DATE = "2024-05-14"

The release schedule on "whattrainisitnow" is maintained by relman and is accessible via an API. Instead of manually looking up these dates and hard-coding them into Shipit, I propose querying this API. This automation will not only ensure consistency between "whattrainisitnow" and Shipit but will also reduce the need for manual updates and deployments related to these dates.

Implementing these changes will help the progress of issue #1418.