Bundled Themes: Correct `@since` tags to refer to the theme version instead of WordPress version.

This makes `@since` notations in Twenty Seventeen, Twenty Nineteen, and Twenty Twenty consistent with the previous bundled themes.

See #48303.
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git-svn-id: https://core.svn.wordpress.org/trunk@46627 1a063a9b-81f0-0310-95a4-ce76da25c4cd