fix: take past quarters into account for updates

[mstimberg]

The current logic to apply updates was a bit surprising to me when I applied it to a repository that I wanted to convert to SPEC-0 dependencies: it applies updates to minimal dependencies from the current quarter but does not take previous quarters into account. For example, a project depending on pandas>=2.1, will not see its minimum dependency updated, even though it is out of date, since the update to use pandas>=2.2 already dates from the previous quarter. This does not matter for the ongoing usage of this action, once SPEC-0 is already in place, but I think new users would expect that it updates all outdated dependencies on its first usage (as e.g. dependabot would do).

With this PR, it scans through the schedule and uses the most recent applicable minimum dependency for each package. This requires a change to the test suite as well, since the previous test relied on the fact that it would always update based on the very first schedule entry, regardless of the current date.