Show the nag we show for a stuck .maintenance file when an auto update failed.

see #25654.

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Make some changes to the email templates to account for the fact that an auto update offer might not be to the latest version.

For example, 3.7.2 install is served a 3.7.3 update, but the current version is 3.8.1.

This commit also allows for core update notification emails to be sent even when we are unable to run an auto update due to filesystem configuration (VCS checkout or file permissions). But, since these emails go through WP_Automatic_Updater, they will not be sent if the updater is outright disabled.

fixes #25654.

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Have heartbeat's connection lost notice handle 503 errors send by the upgrader's maintenance mode.

fixes #25660 for 3.7.

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3.7-RC2

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Trunk is now 3.8-alpha

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Pinking shears
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