Release Posts Automation: short-circuit release post assembly unless an appropriate date

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Related: #34453 (closed)

In the %16.6 milestone, we are planning to change our release cadence from our usual 22nd of the month to more flexible 3rd Thursday of the month. As part of that transition, we need to update our release post automation for assembly, deprecation indexing, and kickoff tasks to work work with relative dates.

We can use the canonical releases.yml file as the source of truth for release information.

• add a release finder class to get the current release
• add a method to determine if the relative date from release is the assembly day
• add short-circuit for ReleasePost::Builder.auto_build to exit early if not 4 days before the release date

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