Various improvements to version update checks (#15517)

* Various improvements to version update checks

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* Release project changes *
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Updating version_info.clj code to build version-info-ee.json file for Enterprise Edition build

Adding test for both oss and ee version info file generation logic

Decoupling GitHub milestone from version and setting it to be the OSS value for :ee release build

Adding nREPL alias for release deps.edn project and usage note

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* Core product changes *
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Adding new :ee version of :mb-version-info-url (:mb-version-info-ee-url) with default value to match the file name generated by the release code

Making enterprise-edition? public from core.clj, since it's now needed from the upgrade check code to switch between the URLs

Adding new setting to store when updates were last checked (since it's on a 12-hour interval)

Changing update check code to both set this new timestamp setting (always) to the current time, and set version-info itself to nil if the update check fails (so that users won't see "latest and greatest" if the update check succeeded at some point in the past, but then started failing

Frontend changes:

Showing the "last checked" timestamp (in user's browser timezone) when we show the "Sorry" text because of being unable to check for updates, so they know when the last check was made

Removing custom implementation of MetabaseSettings.versionInfo from EE frontend code since it's no longer needed (and never worked), because the EE backend will now be hitting a different URL with the version info specifically for EE releases

Linking to https://www.metabase.com/docs/<newVersion>/operations-guide/upgrading-metabase.html URL instead of simply /start