Update maintenance mode message wording

[dknauss]

Trac ticket: https://core.trac.wordpress.org/ticket/35530

Summary

• Updates the default maintenance-mode message from “Check back in a minute.” to “Check back shortly.”

Why

The default maintenance response sends Retry-After: 600, and wp_is_maintenance_mode() can continue blocking normal boot while the timestamped .maintenance lockfile is less than 10 minutes old. In normal successful updates the interruption is usually much shorter, but in common interrupted-update cases “in a minute” can be objectively misleading.

“Shortly” keeps the message concise without promising a specific duration.

Testing

• php -l src/wp-includes/load.php

[github-actions]

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[github-actions]

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