Adds styles into `wp-includes/css/classic-themes.css` to restore file block button element styling for classic themes.
Why?
In [54257] and [54118], button element styles were moved from each block to `theme.json`. For the file block, the download button styles were removed from the block's stylesheet. These changes impacted themes without a `theme.json` file (i.e. classic themes).
This changeset restores the styles for backwards-compatibility.
References:
* [https://github.com/WordPress/gutenberg/pull/47686 Gutenberg PR 47686]
Follow-up to [54257], [54118].
Props wildworks, scruffian, mamaduka, ntsekouras.
Fixes#57688.
Built from https://develop.svn.wordpress.org/trunk@55323
git-svn-id: http://core.svn.wordpress.org/trunk@54856 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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