Update deprecate date format

[samford]

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There was internal discussion about the varying date formats used for deprecate! :date => "...". At present, there are two formats in use: "January 1, 2020" and "2020-01-01" (ISO-8601).

There's a vocal contingent among us who are strongly in favor of ISO-8601 and believe there's value in this format over others, so this PR updates the existing deprecations accordingly.

[issyl0]

The one true date format!

[Bo98]

This passes the syntax checks so should be good to merge and relieve the load on the CI queue.

[Bo98]

(and it also doesn't require new bottles)

[samford]

This passes the syntax checks so should be good to merge and relieve the load on the CI queue.

I was thinking that was the way to go but I was unsure. Thanks, Bo.

[chenrui333]

❤️