Fix documentation around Expires header

[menelaos]

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• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddocks for new, public APIs

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• Update the Changelog.md file with a link to your PR
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[menelaos]

RFC 2616 has been obsoleted by RFC 7234, which in turn has been obsoleted by RFC 9111.

Side note:
Initially, the expiry date was set to the year 2037. This was changed in #870.

The newer standard theoretically permits longer lifetimes but I think it doesn't hurt to keep the current behaviour. The relevant passage:

Historically, HTTP required the Expires field value to be no more than a year in the future. While longer freshness lifetimes are no longer prohibited, extremely large values have been demonstrated to cause problems (e.g., clock overflows due to use of 32-bit integers for time values), and many caches will evict a response far sooner than that.

[menelaos]

The current CI failure should be unrelated to the changes here.